U.S. patent application number 13/496560 was filed with the patent office on 2012-08-09 for communication system, relay device, management server, and communication terminal.
This patent application is currently assigned to SONY CORPORATION. Invention is credited to Ryo Sawai.
Application Number | 20120201190 13/496560 |
Document ID | / |
Family ID | 43795709 |
Filed Date | 2012-08-09 |
United States Patent
Application |
20120201190 |
Kind Code |
A1 |
Sawai; Ryo |
August 9, 2012 |
COMMUNICATION SYSTEM, RELAY DEVICE, MANAGEMENT SERVER, AND
COMMUNICATION TERMINAL
Abstract
Provided is a communication system including a relay device that
relays communication between a base station and a communication
terminal, and a management server, the management server including
a receiving unit that receives from each base station information
about a communication terminal belonging to the base station and
about the relay device, and a determination unit that determines,
on the basis of the information received from each base station by
the receiving unit, a relay device that is performing communication
interfering with communication in an adjacent cell. When the relay
device is determined as a relay device that is performing
communication interfering with the communication in the adjacent
cell, the relay device determines an interference avoidance control
and executes the determined interference avoidance control.
Inventors: |
Sawai; Ryo; (Tokyo,
JP) |
Assignee: |
SONY CORPORATION
TOKYO
JP
|
Family ID: |
43795709 |
Appl. No.: |
13/496560 |
Filed: |
August 3, 2010 |
PCT Filed: |
August 3, 2010 |
PCT NO: |
PCT/JP10/63069 |
371 Date: |
April 24, 2012 |
Current U.S.
Class: |
370/315 |
Current CPC
Class: |
H04B 7/2606 20130101;
H04W 16/26 20130101; H04W 24/00 20130101; H04L 1/0026 20130101;
H04L 2001/0097 20130101; H04B 7/15542 20130101; H04L 1/06 20130101;
H04W 84/047 20130101; H04W 28/04 20130101 |
Class at
Publication: |
370/315 |
International
Class: |
H04B 7/14 20060101
H04B007/14; H04W 36/00 20090101 H04W036/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 25, 2009 |
JP |
2009-220480 |
Feb 25, 2010 |
JP |
2010-040225 |
Claims
1. A communication system comprising: a relay device that relays
communication between a base station and a communication terminal;
and a management server, the management server including a
receiving unit that receives from each base station information
about a communication terminal belonging to the base station and
about the relay device, and a determination unit that determines,
on the basis of the information received from each base station by
the receiving unit, a relay device that is performing communication
interfering with communication in an adjacent cell, wherein when
the relay device is determined as a relay device that is performing
communication interfering with the communication in the adjacent
cell, the relay device determines an interference avoidance control
and executes the determined interference avoidance control.
2. The communication system according to claim 1, wherein the relay
device determines as the interference avoidance control a handover
to a base station that forms the adjacent cell.
3. The communication system according to claim 1, wherein the relay
device determines as the interference avoidance control a handover
of a communication terminal belonging to the relay device to a
relay device belonging to the adjacent cell.
4. The communication system according to claim 3, wherein the relay
device sets as a target to be handed over a communication terminal
whose communication quality does not meet a predetermined criteria
among communication terminals belonging to the relay device.
5. The communication system according to claim 4, wherein the relay
device executes the handover when the number of the communication
terminals belonging to the relay device is greater than or equal to
a predetermined number.
6. A relay device, wherein the relay device: receives from each
base station information about a communication terminal belonging
to the base station and about the relay device; and determines,
when the relay device is determined as a relay device that is
performing communication interfering with communication in an
adjacent cell by a management server that determines a relay device
that is performing communication interfering with the communication
in the adjacent cell on the basis of the information received from
each base station, an interference avoidance control, and executes
the determined interference avoidance control.
7. A management server comprising: a receiving unit that receives
from each base station information about a communication terminal
belonging to the base station and about a relay device that relays
communication between the base station and the communication
terminal; and a determination unit that determines, on the basis of
the information received from each base station by the receiving
unit, a relay device that is performing communication interfering
with communication in an adjacent cell, wherein the relay device,
which is determined by the determination unit as a relay device
that is performing communication interfering with the communication
in the adjacent cell, determines an interference avoidance control
and executes the determined interference avoidance control.
8. A communication terminal, wherein the communication terminal
communicates with a base station via a relay device that determines
an interference avoidance control and executes the determined
interference avoidance control when the relay device is determined
as a relay device that is performing communication interfering with
communication in an adjacent cell by a management server that
receives from each base station information about a communication
terminal belonging to the base station and about a relay device and
determines a relay device that is performing communication
interfering with the communication in the adjacent cell on the
basis of the information received from each base station.
9. A communication system comprising: a small-to-medium-sized base
station that communicates with a communication terminal; and a
management server, the management server including a receiving unit
that receives information about a communication terminal belonging
to the small-to-medium-sized base station, and a determination unit
that determines, on the basis of the information received by the
receiving unit, a small-to-medium-sized base station that is
performing communication interfering with another communication,
wherein when the small-to-medium-sized base station is determined
by the determination unit as a small-to-medium-sized base station
that is performing communication interfering with another
communication, the relay device determines an interference
avoidance control and executes the determined interference
avoidance control.
Description
TECHNICAL FIELD
[0001] The present invention relates to a communication system, a
relay device, a management server, and a communication
terminal.
BACKGROUND ART
[0002] In IEEE (Institute of Electrical and Electronics Engineers)
802.16j, a relay technology is standardized. In addition, in 3GPP
(Third Generation Partnership Project) LTE-A (Long Term Evolution
Advanced), a technology of using a relay device (RN: Relay node) is
also actively studied in order to realize an improvement in the
throughput of a communication terminal (UE: User Equipment) located
at a cell edge.
[0003] Such a relay device, upon receiving a signal transmitted
from a base station in a downlink, amplifies the signal and
transmits the amplified signal to a communication terminal. By
performing such relay, the relay device can increase the
signal-to-noise ratio compared to when a signal is transmitted
directly from the base station to the communication terminal.
Likewise, in an uplink, the relay device can also maintain the high
signal-to-noise ratio by relaying a signal transmitted from the
communication terminal to the base station. Such a relay device is
described in, for example, Non-Patent Literature 1 to 3.
CITATION LIST
Non-Patent Literature
[0004] Non-Patent Literature 1: R1-090015, "Consideration on
Relay.ppt", China Potevio, CATT, January 2009
[0005] Non-Patent Literature 2: R1-090065, "Joint analog network
coding and Relay", Alcatel-Lucent, January 2009
[0006] Non-Patent Literature 3: R1-091803, "Understanding on Type 1
and Type 2 Relay", Huawei, May 2009
SUMMARY OF INVENTION
Technical Problem
[0007] Herein, it is assumed that most parts of the communication
between the relay device and the communication terminal are
subjected to centralized control by the base station or a
management server that is connected to a plurality of base
stations. However, when a load on the base station or the
management server is considered, it is desired that the
communication between the relay device and the communication
terminal be subjected to decentralized control.
[0008] The present invention has been made in view of the foregoing
problems, and it is an object of the present invention to provide a
communication system, a relay device, a management server, and a
communication terminal that are novel and improved, and are capable
of realizing autonomous interference avoidance control by a
small-to-medium-sized base station such as a relay device.
Solution to Problem
[0009] In order to solve the aforementioned problem, according to
one aspect of the present invention, there is provided a
communication system including a relay device that relays
communication between a base station and a communication terminal,
and a management server, the management server including a
receiving unit that receives from each base station information
about a communication terminal belonging to the base station and
about the relay device, and a determination unit that determines,
on the basis of the information received from each base station by
the receiving unit, a relay device that is performing communication
interfering with communication in an adjacent cell. When the relay
device is determined as a relay device that is performing
communication interfering with the communication in the adjacent
cell, the relay device determines an interference avoidance control
and executes the determined interference avoidance control.
[0010] The relay device may determine as the interference avoidance
control a handover to a base station that forms the adjacent
cell.
[0011] The relay device may determine as the interference avoidance
control a handover of a communication terminal belonging to the
relay device to a relay device belonging to the adjacent cell.
[0012] The relay device may set as a target to be handed over a
communication terminal whose communication quality does not meet a
predetermined criteria among communication terminals belonging to
the relay device.
[0013] The relay device may execute the handover when the number of
the communication terminals belonging to the relay device is
greater than or equal to a predetermined number.
[0014] In order to solve the aforementioned problem, according to
another aspect of the present invention, there is provided a relay
device that receives from each base station information about a
communication terminal belonging to the base station and about the
relay device, and determines, when the relay device is determined
as a relay device that is performing communication interfering with
communication in an adjacent cell by a management server that
determines a relay device that is performing communication
interfering with the communication in the adjacent cell on the
basis of the information received from each base station, an
interference avoidance control, and executes the determined
interference avoidance control.
[0015] In order to solve the aforementioned problem, according to
still another aspect of the present invention, there is provided a
management server including a receiving unit that receives from
each base station information about a communication terminal
belonging to the base station and about a relay device that relays
communication between the base station and the communication
terminal, and a determination unit that determines, on the basis of
the information received from each base station by the receiving
unit, a relay device that is performing communication interfering
with communication in an adjacent cell. The relay device, which is
determined by the determination unit as a relay device that is
performing communication interfering with the communication in the
adjacent cell, determines an interference avoidance control and
executes the determined interference avoidance control.
[0016] In order to solve the aforementioned problem, according to
further another aspect of the present invention, there is provided
a communication terminal. The communication terminal communicates
with a base station via a relay device that determines an
interference avoidance control and executes the determined
interference avoidance control when the relay device is determined
as a relay device that is performing communication interfering with
communication in an adjacent cell by a management server that
receives from each base station information about a communication
terminal belonging to the base station and about a relay device and
determines a relay device that is performing communication
interfering with the communication in the adjacent cell on the
basis of the information received from each base station. In order
to solve the aforementioned problem, according to yet another
aspect of the present invention, there is provided a communication
system including a small-to-medium-sized base station that
communicates with a communication terminal, and a management
server, the management server including a receiving unit that
receives information about a communication terminal belonging to
the small-to-medium-sized base station, and a determination unit
that determines, on the basis of the information received by the
receiving unit, a small-to-medium-sized base station that is
performing communication interfering with another communication.
When the small-to-medium-sized base station is determined by the
determination unit as a small-to-medium-sized base station that is
performing communication interfering with another communication,
the relay device determines an interference avoidance control and
executes the determined interference avoidance control.
Advantageous Effects of Invention
[0017] As described above, according to the present invention, it
is possible to realize autonomous interference avoidance control by
a small-to-medium-sized base station such as a relay device.
BRIEF DESCRIPTION OF DRAWINGS
[0018] FIG. 1 is an explanatory diagram showing the configuration
of a communication system in accordance with an embodiment of the
present invention.
[0019] FIG. 2 is an explanatory diagram showing exemplary resource
allocation when the same frequency is used in an UL and a DL.
[0020] FIG. 3 is an explanatory diagram showing exemplary resource
allocation when different frequencies are used in an UL and a
DL.
[0021] FIG. 4 is an explanatory diagram showing an exemplary format
of a DL radio frame.
[0022] FIG. 5 is an explanatory diagram showing an exemplary format
of an UL radio frame.
[0023] FIG. 6 is an explanatory diagram showing a connection
process sequence.
[0024] FIG. 7 is an explanatory diagram showing a specific example
of a MBSFN transmission/reception process.
[0025] FIG. 8 is an explanatory diagram showing exemplary frequency
allocation to each cell.
[0026] FIG. 9 is a functional diagram showing the configuration of
a communication terminal.
[0027] FIG. 10 is a functional diagram showing the configuration of
a relay device.
[0028] FIG. 11 is a functional block diagram showing the
configuration of a base station.
[0029] FIG. 12 is a functional block diagram showing the
configuration of a management server.
[0030] FIG. 13 is an explanatory diagram showing the influence of
the frequency selective fading.
[0031] FIG. 14 is an explanatory diagram showing the influence of
the frequency selective fading.
[0032] FIG. 15 is an explanatory diagram showing an LTE network
configuration.
[0033] FIG. 16 is an explanatory diagram showing the procedures of
handover between base stations.
[0034] FIG. 17 is a sequence diagram showing the connection
procedures of a communication terminal and a relay device.
[0035] FIG. 18 is a sequence diagram showing the procedures for
handing over a relay device.
[0036] FIG. 19 is a sequence diagram showing the procedures for
handing over a communication terminal.
[0037] FIG. 20 is a sequence diagram showing the procedures for
handing over a communication terminal.
[0038] FIG. 21 is an explanatory diagram showing a specific example
of the decision of the transmission power.
[0039] FIG. 22 is an explanatory diagram showing a specific example
of the decision of beam forming.
[0040] FIG. 23 is an explanatory diagram showing a specific example
of the decision of the transmission timing, insertion of a
non-transmission section, and the like.
[0041] FIG. 24 is an explanatory diagram showing a specific example
of the decision of the transmission timing, insertion of a
non-transmission section, and the like.
[0042] FIG. 25 is an explanatory diagram showing a specific example
of the decision of the transmission timing, insertion of a
non-transmission section, and the like.
[0043] FIG. 26 is an explanatory diagram showing a specific example
of handover of a relay device.
[0044] FIG. 27 is a sequence diagram showing a variation of the
connection procedures of a communication terminal and a relay
device.
[0045] FIG. 28 is a sequence diagram showing the procedures for
handing over a relay device.
[0046] FIG. 29 is an explanatory diagram showing a specific example
of handover of a communication terminal.
[0047] FIG. 30 is a sequence diagram showing the procedures for
handing over a communication terminal.
[0048] FIG. 31 is a sequence diagram showing the procedures for
handing over a communication terminal.
[0049] FIG. 32 is an explanatory diagram showing a configuration
example of a heterogeneous network.
[0050] FIG. 33 is an explanatory diagram showing an overview of a
small-to-medium-sized base station.
[0051] FIG. 34 is an explanatory diagram showing an interference
model in a heterogeneous network.
[0052] FIG. 35 is an explanatory diagram showing exemplary
interference avoidance performed through handover.
[0053] FIG. 36 is an explanatory diagram showing exemplary
interference avoidance performed through beam forming.
[0054] FIG. 37 is an explanatory diagram showing exemplary
interference avoidance performed through transmission power
control.
DESCRIPTION OF EMBODIMENTS
[0055] Hereinafter, preferred embodiments of the present invention
will be described in detail with reference to the appended
drawings. Note that, in this specification and the drawings,
structural elements that have substantially the same function and
structure are denoted by the same reference signs, and repeated
explanation is omitted.
[0056] In addition, in this specification and the drawings, a
plurality of structural elements that have substantially the same
function and structure and are denoted by the same reference signs
may be followed by different alphabets for distinction purposes.
For example, a plurality of structures that have substantially the
same function and structure are distinguished as communication
terminals 20A, 20B, and 20C as needed. However, when there is no
need to particularly distinguish between each of the plurality of
structural elements that have substantially the same function and
structure, only reference signs are assigned. For example, when
there is no need to particularly distinguish between the
communication terminals 20A, 20B, and 20C, they are simply referred
to as communication terminals 20.
[0057] The "Description of Embodiments" will be described in
accordance with the following item order.
[0058] 1. Basic Configuration of the Communication System [0059]
(Exemplary Resource Allocation to Each Link) [0060] (Exemplary
Format of Radio Frame) [0061] (Connection Process Sequence) [0062]
(MBSFN) [0063] (Exemplary Frequency Allocation to Each Cell)
[0064] 2. Specific Configuration of the Communication System [0065]
2-1. Configuration of the Communication Terminal [0066] 2-2.
Configuration of the Relay Device [0067] 2-3. Configuration of the
Base Station
[0068] 3. Control Range A: Centralized Control by the Management
Server
[0069] 4. Control Ranges B and C: Autonomous Control by the Relay
Device
[0070] 5. Other Examples of Application of the Present
Invention
[0071] 6. Conclusion
[0072] <1. Basic Configuration of the Communication
System>
[0073] First, the basic configuration of a communication system 1
in accordance with an embodiment of the present invention will be
described with reference to FIGS. 1 to 8. FIG. 1 is an explanatory
diagram showing the configuration of the communication system 1 in
accordance with an embodiment of the present invention. As shown in
FIG. 1, the communication system 1 in accordance with an embodiment
of the present invention includes base stations 10A and 10B, a
backbone network 12, communication terminals 20A, 20B, and 20X, and
relay devices 30A and 30B.
[0074] The base station 10 manages the communication between the
relay device 30 and the communication terminal 20 existing in a
cell that is formed by the base station 10. For example, the base
station 10A manages scheduling information for communicating with
the communication terminal 20X existing in the cell, and
communicates with the communication terminal 20X in accordance with
the scheduling information. In addition, the base station 10A also
manages scheduling information for communicating with the relay
device 30A existing in the cell and scheduling information for the
relay device 30A and the communication terminal 20A to communicate
with each other.
[0075] Note that the management of the scheduling information can
be performed by the joint cooperation of the base station 10 and
the relay device 30, by the joint cooperation of the base station
10, the relay device 30, and the communication terminal 20, or by
the relay device 30.
[0076] The relay device 30 relays the communication between the
base station 10 and the communication terminal 20 in accordance
with the scheduling information managed by the base station 10.
Specifically, the relay device 30, upon receiving a signal
transmitted from the base station 10 in a downlink, transmits a
signal obtained by amplifying the signal to the communication
terminal 20 using the frequency/time in accordance with the
scheduling information. By performing such relay, the relay device
30 can increase the signal-to-noise ratio compared to when a signal
is transmitted directly from the base station 10 to the
communication terminal 20 located near a cell edge.
[0077] Likewise, in an uplink, the relay device 30 can also
maintain the high signal-to-noise ratio by relaying a signal
transmitted from the communication terminal 20 to the base station
10 in accordance with the scheduling information managed by the
base station 10. Although FIG. 1 shows an example in which only the
relay device 30A exists in the cell formed by the base station 10A,
a plurality of relay devices 30 can exist in the cell formed by the
base station 10A.
[0078] As the types of such relay device 30, Type 1 and Type 2 have
been proposed. The relay device 30 of Type 1 has an individual cell
ID and is permitted to operate its own cell. Thus, the relay device
30 of Type 1 operates in such a way that it is recognized as the
base station 10 by the communication terminal 20. However, the
relay device 30 of Type 1 operates not entirely autonomously, and
performs relay communication within the range of resources that are
allocated by the base station 10.
[0079] Meanwhile, the relay device 30 of Type 2 does not have an
individual cell ID unlike Type 1, and assists in the direct
communication between the base station 10 and the communication
terminal 20. For example, relay transmission technologies using
Cooperative relay and Network coding have been studied. The
characteristics of Type 1 and Type 2 that are currently studied are
shown in Table 1 below.
TABLE-US-00001 TABLE 1 Item Type 1 Type 2 Decision R1-091098
R1-091632 Type of Relay L2 and L3 Relay L2 PHY Cell ID Own cell ID
No cell ID Transparency Non transparent Relay Transparent Relay
node to node to UE UE New cell Create new cell (another Not create
new cell eNB) RF parameters Optimized parameters N/A HO Inter cell
HO (generic HO transparently to UE HO) Control Channel Generate
synch. channel, Not generate its own Generation RS, H-ARQ channel
and channel but decodes/ scheduling information forwards donor
eNB's etc. signal to UE Backward Support (appear as a Support (able
to relay also compatibility Rel-8 eNB to Rel-8 UE) to/from Rel-8
UE) LTE-A (Forward Support (it appear -- compatibility) differently
than Rel-8 eNB to LTE-A UE) Awareness to MS -- (>Rel-8 eNB to --
LTE-A UEs or Relay) Cooperation Inter cell cooperation Intra cell
cooperation Backhaul Higher Lower utilization Usage model Coverage
extension Throughput enhancement and coverage extension Cost Higher
Lower
[0080] As described above, the communication terminal 20
communicates with the base station 10 either directly or via the
relay device 30 in accordance with the scheduling information
managed by the base station 10. Note that examples of data that are
transmitted/received by the communication terminal 20 include voice
data; music data such as music, lectures, or radio programs; still
image data such as photographs, documents, paintings, or charts;
and moving image data such as movies, television programs, video
programs, or game images. The communication terminal 20 can be an
information processing device having a wireless communication
function such as a portable phone or a PC (Personal computer).
[0081] The management server 16 is connected to each base station
10 via the backbone network 12. The management server 16 has a
function of an MME (Mobile Management Entity). In addition, the
management server 16 can also have a function of a serving gateway.
The management server 16 receives from each base station 10
management information indicating the state of a cell formed by
each base station 10, and controls communication in the cell formed
by each base station 10 on the basis of the management information.
Note that the function of the management server 16 can be
implemented with a plurality of physically separated
configurations.
[0082] (Exemplary Resource Allocation to Each Link)
[0083] Herein, resource allocation to each link will be described.
Note that, hereinafter, the communication channel between the base
station 10 and the relay device 30 will be referred to as a relay
link, the communication channel between the relay device 30 and the
communication terminal 20 will be referred to as an access link,
and the direct communication channel between the base station 10
and the communication terminal 20 will be referred to as a direct
link. In addition, the communication channel toward the base
station 10 will be referred to as an UL (uplink), and the
communication channel toward the communication terminal 20 will be
referred to as a DL (downlink). Note also that communication
through each link is performed on the basis of OFDMA.
[0084] The relay device 30, in order to prevent mutual interference
between the relay link and the access link, separates the relay
link and the access link from each other on the basis of the
frequency or time. For example, the relay device 30 can separate
the relay link and the access link in the same direction from each
other on the basis of TDD (Time Division Duplexing) using a common
frequency.
[0085] FIG. 2 is an explanatory diagram showing exemplary resource
allocation when the same frequency is used in the UL and the DL. As
shown in FIG. 2, a radio frame includes a sub-frame 0 to a
sub-frame 9. In the example shown in FIG. 2, the relay device 30,
in accordance with an instruction from the base station 10,
recognizes the sub-frames 8 and 9 as the resources for the DL of
the access link, and relays a signal transmitted from the base
station 10 to the communication terminal 20 using the sub-frames 8
and 9.
[0086] Note that a PSC (Primary Synchronization Channel) and a SSC
(Secondary Synchronization Channel) that are synchronization
signals for the downlink, and a PBCH (Physical Broadcast CHannel)
are allocated to the sub-frames 0 and 5. In addition, paging
channels are assigned to the sub-frames 1 and 6.
[0087] FIG. 3 is an explanatory diagram showing exemplary resource
allocation when different frequencies are used in the UL and the
DL. As shown in FIG. 3, a frequency f0 is used for the DL and a
frequency f1 is used for the UL. In the example shown in FIG. 3,
the relay device 30, in accordance with an instruction from the
base station 10, recognizes sub-frames 6 to 8 of the frequency f0
as the resources for the DL of the access link, and relays a signal
transmitted from the base station 10 to the communication terminal
20 using the sub-frames 6 to 8 of the frequency f0.
[0088] Note that a PSC and an SSC that are synchronization signals
for the downlink are assigned to the sub-frames 0 and 5 of the
frequency f0 (for the DL), and paging channels are assigned to the
sub-frame 4 and the sub-frame 9.
[0089] (Exemplary Format of Radio Frame)
[0090] Next, a specific exemplary frame format of each of a DL
radio frame and an UL radio frame will be described with reference
to FIG. 4 and FIG. 5.
[0091] FIG. 4 is an explanatory diagram showing an exemplary format
of a DL radio frame. The DL radio frame includes sub-frames 0 to 9,
and each sub-frame includes two 0.5 ms slots. Each 0.5 ms slot
includes seven OFDM (Orthogonal Frequency Division Multiplexing)
symbols.
[0092] As shown in FIG. 4, in the head 1 to 3 OFDM symbols of each
sub-frame, control channels such as PCFICH (Physical Control Format
Indicator CHannel), PHICH (Physical Hybrid ARQ Indicator CHannel),
and PDCCH (Physical Downlink Control CHannel) and arranged.
[0093] Note that each of the aforementioned channels includes the
following information as an example.
[0094] PCFICH: the number of symbols of PDCCH related to Layer 1
and Layer 2
[0095] PHICH: ACK/NACK in response to PUSCH
[0096] PDCCH: downlink control information, scheduling information
for PDSCH/PUSC (the format of a modulation method, encoding ratio,
or the like)
[0097] In addition, one resource block (1 RB), which is the minimum
unit of resource allocation, includes six or seven OFDM symbols and
12 sub-carriers as shown in FIG. 4. A demodulation reference (a
reference signal) is arranged in part of the resource block.
[0098] Further, SSC, PBCH, and PSC are arranged in the sub-frames 0
and 5. Furthermore, a free portion in the radio frame shown in FIG.
4 is used as a PDSCH (Physical Downlink Shared CHannel).
[0099] FIG. 5 is an explanatory diagram showing an exemplary format
of the UL radio frame. Like the DL radio frame, the UL radio frame
includes sub-frames 0 to 9, and each sub-frame includes two 0.5 ms
slots. Each 0.5 ms slot includes seven OFDM symbols.
[0100] As shown in FIG. 5, a demodulation reference (a reference
signal) is arranged in each of the 0.5 ms slots, and CQI
measurement references are arranged in a dispersed manner. The base
station 10 or the relay device 30 on the receiving side performs
channel estimation using the demodulation reference, and
demodulates a received signal in accordance with the channel
estimation result. In addition, the base station 10 or the relay
device 30 on the receiving side acquires CQI between the base
station 10 or the relay device 30 and the relay device 30 or the
communication terminal 20 on the transmitting side by measuring the
CQI measurement reference.
[0101] Further, a free portion in the radio frame shown in FIG. 5
is used as a PUSCH (Physical Uplink Shared CHannel). Note that,
when a CQI report is requested, the communication terminal 20 or
the relay device 30 transmits the CQI report using the PUSCH.
[0102] (Connection Process Sequence)
[0103] Next, a process sequence for connecting the relay device 30
or the communication terminal 20 and the base station 10 will be
described with reference to FIG. 6.
[0104] FIG. 6 is an explanatory diagram showing a connection
process sequence. First, as shown in FIG. 6, the relay device 30 or
the communication terminal 20 transmits an RACH (Random Access
CHannel) preamble to the base station 10 (S62). The base station
10, upon receiving the RACH preamble, acquires TA (Timing Advance)
information, and transmits the TA information together with
allocation resource information to the relay device 30 or the
communication terminal 20 (S64). If the base station 10 is able to
grasp the transmission timing of the RACH preamble, for example,
the base station 10 can acquire as the TA information the
difference between the transmission timing and the reception timing
of the RACH preamble.
[0105] After that, the relay device 30 or the communication
terminal 20 transmits an RRC connection request to the base station
10 using resources indicated by the allocation resource information
(S66). The base station 10, upon receiving the RRC connection
request, transmits an RRC connection resolution indicating the
source of transmission of the RRC connection request (S68).
Accordingly, the relay device 30 or the communication terminal 20
is able to check if the base station 10 has received the RRC
connection request or not.
[0106] Next, the base station 10 transmits to the management server
16, which has a function of an MME, a connection request indicating
that the relay device 30 or the communication terminal 20 is
requesting a service (S70). The management server 16, upon
receiving the connection request, transmits information for
performing setup on the relay device 30 or the communication
terminal 20 through connection setup (S72).
[0107] Then, the base station 10 transmits RRC connection setup to
the relay device 30 or the communication terminal 20 on the basis
of the connection setup from the management server 16 (S74),
whereupon the relay device 30 or the communication terminal 20
performs a connection setup. After that, the relay device 30 or the
communication terminal 20 transmits to the base station 10 RRC
connection complete indicating that the connection setup is
complete (S76).
[0108] Accordingly, the connection between the relay device 30 or
the communication terminal 20 and the base station 10 is completed,
whereby they become able to communicate with each other. Note that
the aforementioned connection process sequence is only exemplary,
and the relay device 30 or the communication terminal 20 and the
base station 10 can be connected through another sequence.
[0109] (MBSFN)
[0110] Next, MBSFN (Multi-media Broadcasting Single Frequency
Network) transmission performed by the base station 10, and an
exemplary operation of the relay device 30 performed in response to
the MBSFN transmission will be described.
[0111] MBSFN is a mode in which a plurality of base stations 10
concurrently performs data broadcast transmission using the same
frequency. Thus, according to MBSFN, the relay device 30 of Type 1,
which virtually operates as a base station, transmits a control
channel and the like for the DL using the same frequency as that of
the base station 10. Hereinafter, a specific flow of the MBSFN
transmission/reception process will be described with reference to
FIG. 7.
[0112] FIG. 7 is an explanatory diagram showing a specific example
of the MBSFN transmission/reception process. First, as shown in
FIG. 7, the base station 10 and the relay device 30 concurrently
transmit PDCCH. Herein, following the PDCCH, the base station 10
transmits R-PDCCH for controlling the relay in addition to the
PDSCH for the communication terminal 20. After the R-PDCCH, PDSCH
(data to be relayed) for the relay device 30 is transmitted. Note
that a non-transmission section is provided after the PDSCH for the
relay device 30.
[0113] The relay device 30, after transmitting the PDCCH, undergoes
a section of switching to a reception process, and receives the
PDSCH (data to be relayed) from the base station 10. Then, the
relay device 30 switches the reception process to a transmission
process in the non-transmission section provided after the PDSCH
(data to be relayed) from the base station 10. Further, the relay
device 30 adds PDCCH to the decoded PDSCH (data to be relayed) in
the next step, and relay-transmits it to the communication terminal
20.
[0114] Accordingly, existing communication terminals, which are not
based on the presence of the relay device 30, can relish the relay
by the relay device 30 without confusion.
[0115] (Exemplary Frequency Allocation to Each Cell)
[0116] Next, exemplary frequency allocation to each cell when a
plurality of cells is adjacent to one another will be
described.
[0117] FIG. 8 is an explanatory diagram showing exemplary frequency
allocation to each cell. When each cell includes three sectors,
allocating frequencies f1 to f3 to the respective sectors as shown
in FIG. 8 allows interference of the frequencies at the cell
boundary to be suppressed. Such allocation is particularly
effective in a densely populated area with high traffic.
[0118] Note that in LTE-A, in order to realize high end-to-end
throughput, a variety of new technologies have been studied such as
spectrum aggregation, network MIMO, uplink multiuser MIMO, and
relay technologies. Therefore, with the advent of a new mobile
application with high throughput, there is a possibility that
frequency resources may become depleted even in suburban areas.
Further, in the introduction of LTE-A, there is a possibility that
introduction of the relay device 30 may become activated in order
to realize low-cost infrastructure development.
[0119] <2. Specific Configuration of the Communication
System>
[0120] The basic configuration of the communication system 1 in
accordance with the present embodiment has been described above
with reference to FIG. 1 to FIG. 8. Next, the specific
configuration of the communication system 1 in accordance with the
present embodiment will be described with reference to FIGS. 9 to
11.
[0121] (2-1. Configuration of the Communication Terminal)
[0122] FIG. 9 is a functional diagram showing the configuration of
the communication terminal 20. As shown in FIG. 9, the
communication terminal 20 includes a plurality of antennae 220a to
220n, an analog processing unit 224, an AD/DA converter unit 228,
and a digital processing unit 230.
[0123] Each of the plurality of antennae 220a to 220n receives a
radio signal from the base station 10 or the relay device 30 and
acquires a high-frequency electrical signal, and then supplies the
high-frequency signal to the analog processing unit 224. In
addition, each of the plurality of antennae 220a to 220n transmits
a radio signal to the base station 10 or the relay device 30 on the
basis of a high-frequency signal supplied from the analog
processing unit 224. As the communication terminal 20 has a
plurality of antennae 220a to 220n as described above, it can
perform MIMO (Multiple Input Multiple Output) communication or
diversity communication.
[0124] The analog processing unit 224 converts a high-frequency
signal transmitted from the plurality of antennae 220a to 220n into
a baseband signal by performing analog processing such as
amplification, filtering, or down conversion. In addition, the
analog processing unit 224 converts a baseband signal supplied from
the AD/DA converter unit 228 into a high-frequency signal.
[0125] The AD/DA converter unit 228 converts the baseband signal in
an analog format supplied from the analog processing unit 224 into
a digital format, and supplies it to the digital processing unit
230. In addition, the AD/DA converter unit 228 converts the
baseband signal in a digital format supplied from the digital
processing unit 230 into an analog format, and supplies it to the
analog processing unit 224.
[0126] The digital processing unit 230 includes a synchronizing
unit 232, a decoder 234, an encoder 240, and a control unit 242.
Among them, the synchronizing unit 232, the decoder 234, the
encoder 240, and the like function as a communication unit for
communicating with the base station 10 or the relay device 30,
together with the plurality of antennae 220a to 220n, the analog
processing unit 224, and the AD/DA converter unit 228.
[0127] The synchronizing unit 232 is supplied with a
synchronization signal such as a PSC or a SSC, which has been
transmitted from the base station 10 or the relay device 30, from
the AD/DA converter unit 228, and performs a synchronization
process on a radio frame on the basis of the synchronization
signal. Specifically, the synchronizing unit 232 computes the
correlation between the synchronization signal and a known sequence
pattern, and detects the peak position of the correlation, thereby
synchronizing a radio frame.
[0128] The decoder 234 decodes the baseband signal supplied from
the AD/DA converter unit 228 to obtain the received data. Note that
the decoding can include, for example, a MIMO reception process and
an OFDM demodulation process.
[0129] The encoder 240 encodes the data to be transmitted such as
PUSCH, and supplies it to the AD/DA converter unit 228. Note that
the encoding can include, for example, a MIMO transmission process
and an OFDM modulation process.
[0130] The control unit 242 controls the entire operation in the
communication terminal 20 such as a transmission process, a
reception process, and a process of connecting to the relay device
30 or the base station 10. For example, the communication terminal
20, under the control of the control unit 242, performs a
transmission process and a reception process using resource blocks
allocated by the base station 10. Note that the control unit 242
controls a transmission process in accordance with a transmission
parameter specified by the base station 10 or the relay device 30.
For example, when the base station 10 has specified a TPC (Transmit
Power Control) parameter for the communication terminal 20 using
PDCCH, the control unit 242 controls a transmission process in
accordance with the TPC parameter specified by the base station
10.
[0131] Meanwhile, when the base station 10 or the relay device 30
has requested a CQI report to the communication terminal 20 using
PDCCH, the digital processing unit 230 measures the channel quality
(e.g., received power) using a demodulation reference transmitted
from the base station 10 or the relay device 30. The control unit
242 generates a CQI report on the basis of the aforementioned
measurement result, and supplies the generated CQI report to the
encoder 240. Consequently, the CQI report is transmitted to the
base station 10 or the relay device 30 using PUSCH.
[0132] (2-2. Configuration of the Relay Device)
[0133] Next, the configuration of the relay device 30 will be
described with reference to FIG. 10.
[0134] FIG. 10 is a functional block diagram showing the
configuration of the relay device 30. As shown in FIG. 10, the
relay device 30 includes a plurality of antennae 320a to 320n, an
analog processing unit 324, an AD/DA converter unit 328, and a
digital processing unit 330.
[0135] Each of the plurality of antennae 320a to 320n receives a
radio signal from the base station 10 or the communication terminal
20 and acquires a high-frequency electrical signal, and then
supplies the high-frequency signal to the analog processing unit
324. In addition, each of the plurality of antennae 320a to 320n
transmits a radio signal to the base station 10 or the
communication terminal 20 on the basis of a high-frequency signal
supplied from the analog processing unit 324. As the relay device
30 has a plurality of antennae 320a to 320n as described above, it
can perform MIMO communication or diversity communication.
[0136] The analog processing unit 324 converts a high-frequency
signal supplied from the plurality of antennae 320a to 320n into a
baseband signal by performing analog processing such as
amplification, filtering, or down conversion. In addition, the
analog processing unit 324 converts a baseband signal supplied from
the AD/DA converter unit 328 into a high-frequency signal.
[0137] The AD/DA converter unit 328 converts the baseband signal in
an analog format supplied from the analog processing unit 324 into
a digital format, and supplies it to the digital processing unit
330. In addition, the AD/DA converter unit 328 converts the
baseband signal in a digital format supplied from the digital
processing unit 330 into an analog format, and supplies it to the
analog processing unit 324.
[0138] The digital processing unit 330 includes a synchronizing
unit 332, a decoder 334, a buffer 338, an encoder 340, and a
control unit 342. Among them, the synchronizing unit 332, the
decoder 334, the encoder 340, and the like function as a receiving
unit, a transmitting unit, and a relay unit for communicating with
the base station 10 or the communication terminal 20, together with
the plurality of antennae 320a to 320n, the analog processing unit
324, and the AD/DA converter unit 328.
[0139] The synchronizing unit 332 is supplied with a
synchronization signal, which has been transmitted from the base
station 10, from the AD/DA converter unit 328, and performs a
synchronization process on a radio frame on the basis of the
synchronization signal. Specifically, the synchronizing unit 332
computes the correlation between the synchronization signal and a
known sequence pattern, and detects the peak position of the
correlation, thereby synchronizing a radio frame.
[0140] The decoder 334 decodes the baseband signal supplied from
the AD/DA converter unit 328, and obtains relay data addressed to
the base station 10 or to the communication terminal 20. Note that
the decoding can include, for example, a MIMO reception process, an
OFDM demodulation process, and an error correction process.
[0141] The buffer 338 temporally stores the relay data addressed to
the base station 10 or to the communication terminal 20 obtained by
the decoder 334. Then, under the control of the control unit 342,
the relay data addressed to the communication terminal 20 is read
from the buffer 338 into the encoder 340 using resource blocks for
the DL of the access link. Likewise, under the control of the
control unit 342, the relay data addressed to the base station 10
is read from the buffer 338 into the encoder 340 using resource
block for the UL of the relay link.
[0142] The encoder 340 encodes the relay data supplied from the
buffer 338, and supplies it to the AD/DA converter unit 328. Note
that the encoding can include, for example, a MIMO transmission
process and OFDM modulation process.
[0143] The control unit 342 controls the entire operation in the
relay device 30 such as a transmission process, a reception
process, and a process of connecting to the base station 10 or the
communication terminal 20. For example, the relay device 30, under
the control of the control unit 342, performs a transmission
process and a reception process using resource blocks allocated by
the base station 10.
[0144] The range that can be controlled by the control unit 342 is
selected by the base station 10. Specifically, one of the control
ranges A to C is selected by the base station 10, and the control
unit 342 controls the communication in accordance with the control
range selected by the base station 10. The criteria for selecting
the control range with the base station 10 and the details of the
control ranges A to C are described below. Although this
specification mainly describes an example in which the control
range of the control unit 342 is selected by the base station 10,
the control range of the control unit 342 can also be selected by
the management server 16.
[0145] (2-3. Configuration of the Base Station)
[0146] FIG. 11 is a functional block diagram showing the
configuration of the base station 10. As shown in FIG. 11, the base
station 10 includes a plurality of antennae 120a to 120n, an analog
processing unit 124, an AD/DA converter unit 128, a digital
processing unit 130, and a backbone communication unit 146.
[0147] Each of the plurality of antennae 120a to 120n receives a
radio signal from the relay device 30 or the communication terminal
20 and acquires a high-frequency electrical signal, and then
supplies the high-frequency signal to the analog processing unit
124. In addition, each of the plurality of antennae 120a to 120n
transmits a radio signal to the relay device 30 or the
communication terminal 20 on the basis of a high-frequency signal
supplied from the analog processing unit 124. As the base station
10 has a plurality of antennae 120a to 120n as described above, it
can perform MIMO communication or diversity communication.
[0148] The analog processing unit 124 converts a high-frequency
signal supplied from the plurality of antennae 120a to 120n into a
baseband signal by performing analog processing such as
amplification, filtering, or down conversion. In addition, the
analog processing unit 124 converts a baseband signal supplied from
the AD/DA converter unit 128 into a high-frequency signal.
[0149] The AD/DA converter unit 128 converts the baseband signal in
an analog format supplied from the analog processing unit 124 into
a digital format, and supplies it to the digital processing unit
130. In addition, the AD/DA converter unit 128 converts the
baseband signal in a digital format supplied from the digital
processing unit 130 into an analog format, and supplies it to the
analog processing unit 124.
[0150] The digital processing unit 130 includes a decoder 134, an
encoder 140, a control unit 142, a storage unit 144, and a control
range selection unit 148. Among them, the decoder 134, the encoder
140, and the like function as a communication unit for
communicating with the relay device 30 or the communication
terminal 20, together with the plurality of antennae 120a to 120n,
the analog processing unit 124, and the AD/DA converter unit
128.
[0151] The decoder 134 decodes the baseband signal supplied from
the AD/DA converter unit 128 to obtain the received data. Note that
the decoding can include, for example, a MIMO reception process, an
OFDM demodulation process, and an error correction process.
[0152] The encoder 140, for example, encodes PDSCH and supplies it
to the AD/DA converter unit 128. Note that the encoding can
include, for example, a MIMO transmission process and an OFDM
modulation process.
[0153] The control unit 142 controls the entire communication in a
cell formed by the base station 10, such as a transmission process,
a reception process, a process of connecting to the relay device 30
or the communication terminal 20, and management of the scheduling
information. For example, the control unit 142 schedules the relay
link communication between the base station 10 and the relay device
30 and the access link communication between the relay device 30
and the communication terminal 20.
[0154] Further, the control unit 142 causes the storage unit 144 to
hold management information indicating the state of the cell formed
by the base station 10. Examples of the management information are
shown below.
[0155] (1) Information about the location of each relay device 30
and each communication terminal 20 belonging to the base station
10
[0156] (2) ID, Qos class, and scheduling information of each relay
device 30 and each communication terminal 20 belonging to the base
station 10
[0157] (3) Communication quality information of each direct link,
each relay link, and each access link (e.g., CQI information, TPC
communication, or both)
[0158] (4) Permissible interference level of each communication
terminal 20 belonging to the base station 10 (e.g., the difference
between the necessary SNIR on the Qos basis expected for each
communication link and the actually measured SINR)
[0159] Note that the information about the location of the relay
device 30 can include positional information acquired with a GPS,
TA information indicating the distance between the base station 10
and the relay device 30, or information indicating the direction of
the relay device 30. The direction of the relay device 30 can be
acquired with an algorithm that estimates the arrival direction of
a signal transmitted from the relay device 30 or by receiving the
directivity. Similarly, the information about the location of the
communication terminal 20 can include positional information
acquired with a GPS, TA information indicating the distance between
the communication terminal 20 and the relay device 30, or
information indicating the direction of the communication terminal
20.
[0160] The control range selection unit 148 selects a control range
to be granted for the relay device 30 belonging to the base station
10, from among a plurality of control ranges. For example, the
plurality of control ranges includes a control range A (a first
control range) a control range B (a second control range), and a
control range C (a third control range). Hereinafter, each control
range will be briefly described first, and then the criteria for
selecting the control range will be described.
[0161] The control range A includes control that doe not require
"addition" of extra resources by the relay device 30 (e.g., link
adaptation in the range that TPC or addition of resources is
unnecessary), and does not include control that requires a change
or setting of resources. Thus, when the control range A is
selected, most parts of the operation of the relay device 30 are
controlled by the base station 10.
[0162] The control range B includes link adaptation in the range
that addition of resources is necessary, handover of the relay
device 30, and handover of the communication terminal 20 belonging
to the relay device 30. The control range C includes, in addition
to the control range B, flexible resource scheduling for the
communication terminal 20 within the range of the extra resources
allocated by the base station 10. Herein, the resource scheduling
indicates an operation that is necessary for creating a link for a
terminal to be newly connected. For example, in the case of the
control range B, there may be cases in which a handover request or
a receiving operation would not be able to be fully implemented
with the amount of resources allocated in the control range B. In
such cases, it is possible to, for example, allocate further extra
resources to a handover destination, or allocate further extra
resources to the relay device 30 and then change the control range
to C.
[0163] The control range selection unit 148 selects one of the
aforementioned control ranges A to C in accordance with the amount
of traffic in the cell formed by the base station 10. For example,
the control range selection unit 148 can select the control range B
when the amount of traffic falls within a predetermined range,
select the control range A when the amount of traffic is above the
predetermined range, and select the control range C when the amount
of traffic is below the predetermined range.
[0164] Specifically, the control range selection unit 148 can
select the control range A when there is congested traffic and thus
there are no available resources, select the control range B when
the available resources account for less than or equal to 30%, and
select the control range C when the available resources account for
more than 30%.
[0165] Note that the control unit 142, when the control range A is
selected, allocates the minimum required resources to the relay
device 30, but in order to respond to an UL connection request from
the communication terminal 20, preferentially secures resources for
the UL.
[0166] Meanwhile, the control unit 142, when the control range B is
selected, allocates a relatively large amount of resources to the
relay device 30. For example, provided that the amount of resources
that are currently used by the relay device 30 is "10," the control
unit 142 can set the amount of resources allocated to the relay
device 30 to "15." Accordingly, it becomes possible for the relay
device 30 to immediately perform link adaptation that requires new
resources.
[0167] Further, the control unit 142, when the control range C is
selected, allocates extra resources to the relay device 30 in
accordance with the number of communication terminals 20 belonging
to the relay device 30. For example, the control unit 142 can
allocate more extra resources to the relay device 30 that has a
larger number of communication terminals 20 belonging thereto. More
specifically, for example, provided that the amount of extra
resources is "40," a single communication terminal 20 belongs to
the relay device 30A, and three communication terminals 20 belong
to the relay device 30B, the control unit 142 can set the amount of
extra resources allocated to the relay device 30A to "10," and set
the amount of extra resources allocated to the relay device 30B to
"30." Accordingly, it becomes possible for the relay device 30 to
autonomously perform resource scheduling within the range of the
allocated resources. Herein, when access to a given relay device 30
from communication terminals 20 is concentrated, the control unit
142 can cause the communication terminals 20 to be handed over to
the base station 10 or to another relay device 30 for load
distribution purposes.
[0168] Although the description has been made above of an example
in which the control range selection unit 148 selects the control
range in accordance with the amount of traffic, the selection
method is not limited thereto. For example, the control range
selection unit 148 can dynamically select the control range on the
basis of one or any combination of a variety of elements such as a
load on the base station 10, power consumption, the number of
communication terminals 20, information about whether or not the
relay device 30 is disposed temporally for an outdoor event, and
the relationship with another base station.
[0169] The backbone communication unit 146 communicates with the
management server 16 via the backbone network 12. For example, the
backbone communication unit 146 transmits to the management server
16 information indicated by (1) to (4) above stored in the storage
unit 144. In that case, the backbone communication unit 146 can,
regarding (2) above, further transmit reference counter information
for detecting mis-synchronization between the base station 10 and
another base station, by considering a case in which the base
station 10 operates asynchronously with the other base station.
[0170] As described above, the relay device 30 performs control in
accordance with the control range selected by the base station 10.
Therefore, the entire operation of the communication system changes
in accordance with the control range of the relay device 30
selected by the base station 10. Thus, hereinafter, interference
avoidance operations performed when the control range A is selected
or when the control range B or the control range C is selected will
be described in detail.
[0171] <3. Control Range A: Centralized Control by the
Management Server>
[0172] When the base station 10 has selected the control range A,
the relay device 30 is not permitted to perform an autonomous
operation almost at all. Thus, the management server 16 determines
the presence or absence of interference and issues an instruction
to execute interference avoidance control. Hereinafter, the
configuration of such management server 16 will be described. Note
that the present embodiment is based on the following points.
[0173] The relay device 30 uses a direct link, and terminates the
procedures of up to "RRC connection complete" with the base station
10 in accordance with similar procedures to those of the
communication terminal 20, and also determines the sub-cell ID,
reference pattern allocation, and the like.
[0174] The base station 10 and the relay device 30 belonging
thereto are synchronized with each other.
[0175] Grouping information that indicates the relay device 30 and
the communication terminal 20 belonging to the relay device 30 is
given by the base station 10 in advance (the base station 10
determines the necessity of relay from a CQI report or TA
information, and allocates resources for relay if necessary).
[0176] Ptx_DL>>Ptx_RL and Ptx_AL (Ptx: the maximum
transmission power, DL: direct link (direct link between the base
station 10 and the communication terminal 20), AL: access link, and
RL: relay link)
[0177] The primary object to be achieved is to take measures
against interference to the direct link, in particular,
interference to the direct link of the communication device (LTE
UE) that is not based on the presence of the relay device 30.
[0178] FIG. 12 is a functional block diagram showing the
configuration of the management server 16. As shown in FIG. 12, the
management server 16 includes a communication unit 160, a storage
unit 162, an interference determination unit 164, and a base
station management unit 166. Note that the function of the
management server 16 can be implemented on a single base station 10
to realize centralized control or be implemented on a plurality of
base stations 10 to realize autonomous control.
[0179] The communication unit 160 is connected to each base station
10, and has the functions of a receiving unit that receives
information from each base station 10 and a transmitting unit that
transmits information to each base station 10. For example, the
communication unit 160 receives management information indicated by
(1) to (4) above from each base station 10. The management
information received by the communication unit 160 is recorded on
the storage unit 162.
[0180] The interference determination unit 164 determines whether
or not mutual interference would occur between the communications
controlled by different base stations 10, using part or all of the
management information indicated by (1) to (4) above. For example,
the interference determination unit 164 can determine that
interference would occur when the distance between a relay device
30 or a communication terminal 20, which belongs to a given base
station 10, and a relay device 30 or a communication terminal 20,
which belongs to another base station 10, is less than or equal to
a set value. Further, the interference determination unit 164 can
determine that interference would occur when resources, which are
used by each of the pair whose distance is less than or equal to
the set value, overlap each other. Furthermore, the interference
determination unit 164 can determine the presence or absence of
interference on the basis of information from the adjacent base
station 10 or the adjacent relay device 30 obtained by measurement
at the communication terminal 20.
[0181] The base station management unit 166 permits the base
station 10, for which the interference determination unit 164 has
determined that interference would not occur, to perform a typical
autonomous operation until when there is an update to the
scheduling information, an update to the location of the
communication terminal 20, or the like, or until when predetermined
report cycles have elapsed. Meanwhile, the base station management
unit 166 instructs the base station 10, for which the interference
determination unit 164 has determined that interference would
occur, to perform an interference avoidance operation. Interference
avoidance control indicates control by which interference may
possibly be avoided, or control by which interference can be
avoided under given conditions. Hereinafter, such interference
avoidance control will be described.
[0182] (Interference Avoidance Control)
[0183] When there is no congestion of traffic in one of the base
stations 10 that controls the communication for which it has been
determined that interference would occur, which means that the
resource scheduling for one of the base stations 10 can be changed,
the base station management unit 166 issues an instruction to
change the scheduling information of the one of the base stations
10 as the interference avoidance control. Specifically, the base
station management unit 166 can, in the scheduling information of
the one of the base stations 10, change resources allocated to the
communication, for which it has been determined that interference
would occur, to different resources, and transmit the changed
scheduling information to the one of the base stations 10. In this
case, the base station management unit 166 can only send
information about the change in the scheduling. Herein, the base
station management unit 166 not only changes the scheduling
information for the communication between the one of the base
stations 10 and the relay device 30 but also changes the scheduling
information for the communication between the relay device 30 and
the communication terminal 20.
[0184] Further, the base station management unit 166 can also
allocate resources to the communication terminal 20 while avoiding
resource blocks or sub-carriers whose interference components to
the communication terminal 20 are large. This will be described
hereinafter in conjunction with a summary of OFDMA.
[0185] In OFDMA, adjacent base stations perform communication using
carriers with the same center frequency in a densely populated
area. In this case, for the communication with a communication
terminal located at a cell edge where coverage of a plurality of
base stations overlaps, the plurality of base stations use
sub-carriers that are orthogonal to each other or use different
time slots to avoid interference, whereby limited resources are
effectively utilized. Meanwhile, in a non-densely populated area,
there are sufficient available resources in many cases. Thus,
different orthogonal sub-carriers are fixedly allocated to the
respective base stations.
[0186] As described above, when adjacent base stations operate
adjacent cells using sub-carriers that are orthogonal to each
other, there may be cases in which due to a frequency deviation
caused by various factors (e.g., influence of Doppler frequency),
radiated power outside the band would overlap the sub-carriers at
the edges, causing interference. Thus, frequency allocation and
out-of-band suppression filtering are important.
[0187] Alternatively, when adjacent base stations operate adjacent
cells by allocating different time slots, it is important that the
transmission timing be adjusted so that the boundaries of the time
slots are orthogonal to one another other (so that they are within
the GuardIntervals (GI) of the head symbol) on the basis of the
accurate propagation channel delay for the communication terminal
located at the cell edge.
[0188] Herein, the influence of the frequency selective fading will
be described with reference to FIGS. 13 and 14.
[0189] FIGS. 13 and 14 are explanatory diagrams each showing the
influence of the frequency selective fading. As shown in FIG. 3, an
OFDM modulated signal has, even when the transmission power of each
sub-carrier is the same at the time of transmission, variations in
the received power of each sub-carrier at the time of reception due
to the influence of the frequency selective fading. In addition, as
shown in FIG. 14, the magnitudes of interference components differ
from resource block to resource block.
[0190] Thus, when the base station management unit 166 can
recognize the magnitudes of interference components in each
resource block at the communication terminal 20, the base station
management unit 166 can avoid interference by allocating resources
to the communication terminal 20 while avoiding resource blocks
having large interference components. Further, when the base
station management unit 166 can recognize the magnitudes of
interference components in each sub-carrier, the base station
management unit 166 can avoid interference by avoiding the use of
sub-carriers having large interference within the resource block or
lowering the modulation method.
[0191] Note that the base station management unit 166 can, without
changing the scheduling information by itself, inform one of the
base stations 10 of the communication for which it has been
determined that interference would occur, and prompt the one of the
base stations 10 to change the scheduling information.
[0192] Further, the base station management unit 166 can also
issue, as the interference avoidance control, an instruction to
hand over a relay device 30 or a base station 20 belonging to one
of the base stations 10 that controls the communication for which
it has been determined that interference would occur, to the other
base station 10 or to a relay device 30 belonging to the other base
station 10. Note that the base station management unit 166 can also
premise that there are extra available resources for accepting
handover in the other base station 10 or the relay device 30
belonging to the other base station 10.
[0193] For example, if it is determined that interference would be
avoided by handing over the relay device 30 belonging to one of the
base stations 10 to the other base station 10, the base station
management unit 166 instructs the one of the base stations 10 to
perform the handover. In that case, the base station management
unit 166 informs the base station 10 of the ID of the base station
10, which is the handover destination, information for connection,
and the like. In response, a series of operations for handover is
performed. Herein, examples of the information for connection
include the relative distance from the base station 10, which is
the handover destination, and information indicating the
aforementioned resource blocks or sub-carriers having large
interference components. Hereinafter, referring to FIG. 15 to FIG.
18, typical handover procedures and the like will be described
first, and then a specific flow of the handover of the relay device
30 in accordance with the present embodiment will be described.
[0194] FIG. 15 is an explanatory diagram showing an LTE network
configuration. As shown in FIG. 15, the LTE network includes, in
addition to the management server 16 having a function of an MME
and the base stations 10, an S-GW (Serving GW) 18 that manages the
user data. Handover between the base stations in such a network
configuration is performed in accordance with the procedures shown
in FIG. 16.
[0195] FIG. 16 is an explanatory diagram showing the procedures of
handover between base stations. As shown in FIG. 16, when the
communication terminal 20 and the base station 10A are connected,
the base station 10A transmits to the communication terminal 20
context information (Adjacent eNB context information) indicating
the target to be measured such as an adjacent base station 10B
(S404). After that, the communication terminal 20 measures the
radio field intensity or the like of a signal transmitted from the
base station 10B or the like in accordance with the context
information while at the same time communicating with the base
station 10A. Then, the communication terminal 20 reports to the
base station 10A the measurement information (Mesurement report) in
accordance with predetermined cycles or rule (S408). Note that S404
and S408 can be omitted if the base station 10A hands over the
communication terminal 20 based on the forced determination on the
network side.
[0196] After that, the base station 10A requests the base station
10B to accept handover of the communication terminal 20 (S412), and
once the base station 10B has accepted the request (S416), the base
station 10A instructs execution of handover of the communication
terminal 20 (S420). Then, the communication terminal 20 performs a
process of connecting to the base station 10B, and informs the base
station 10B that preparation for handover is ready (S424). The base
station 10B returns ACK in response to the information (S428), and
also reports to the management server 16/S-GW16 that the
communication terminal 20 has been handed over to the base station
10B (S432).
[0197] Although the description has been made above of a case in
which the network side such as the management server 16 or the
S-GW18 determines the execution of handover on the basis of the
measurement information measured with the communication terminal 20
(a case in which the communication terminal 20 cooperatively
operates), a trigger for the handover is not limited to such
example. For example, handover can be performed on the basis of the
forced determination on the network side such as the management
server 16 or the S-GW 18. Alternatively, the communication terminal
20 can voluntarily perform handover by selecting the base station
10 in accordance with the measurement information and performing a
connection process. As a further alternative, the management server
16 can be arranged such that it physically manages a plurality of
base stations 10 (eNB) like MME or S-GW. Moreover, the management
server 16 can be presumed to be included in the base stations 10 so
that information can be logically exchanged using X2 IF between the
plurality of base stations 10.
[0198] Herein, in LTE-A, coordinated transmission between base
stations called CoMP (Cordinated Multipoint Transmission and
reception) and the like have been studied, and there is a high
possibility that each IF (S11 IF, S1-MME IF, or S1-UIF) may be
enhanced, and management may be performed as if a single
communication terminal 20 belongs to a plurality of base stations
10.
[0199] Thus far, a link management method such as handover for
which the presence of the relay device 30 is taken into
consideration has not been discussed specifically. Thus,
hereinafter, a flow of up to the connection of the relay device 30
will be described first, and then the procedures for handing over
the relay device 30 will be described. Note that the following
description is based on the premise that the management server 16
includes the function of the S-GW16.
[0200] FIG. 17 is a sequence diagram showing the connection
procedures of the communication terminal 20 and the relay device
30. As shown in FIG. 17, when the communication terminal 20 and the
base station 10A are connected, the base station 10 transmits to
the communication terminal 20 context information (Adjacent eNB
& RN context information) indicating the target to be measured
such as an adjacent base station or a nearby relay device 30
(S454). After that, the communication terminal 20 measures the
radio field intensity or the like of a signal transmitted from the
relay device 30A or the like in accordance with the context
information while at the same time communicating with the base
station 10A. Then, the communication terminal 20 reports to the
base station 10A the measurement information in accordance with
predetermined cycles or rule (S458).
[0201] After that, the base station 10A reports to the management
server 16 the measurement information (Relay link information
report) of the nearby relay device 30 (S462). Note that the base
station 10A can also report the measurement information of the
adjacent base station at the same time. Then, the management server
16 transmits to the base station 10A a confirmation in response to
the measurement information (S466). Further, the management server
16 determines the information to be used for the communication
terminal 20 to connect to the relay device 30 (e.g., ID of the
relay device 30 to be connected), and transmits it to the base
station 10A (S470). Then, the base station 10A issues, on the basis
of the information received from the management server 16, a relay
request to the target relay device 30 (the relay device 30A in the
example shown in FIG. 17) (S474).
[0202] Next, when the relay device 30A has transmitted to the base
station 10A a confirmation in response to the relay request (S478),
the base station 10A instructs the communication terminal 20 to
connect to the relay device 30A (S482). Herein, the base station
10A can send the ID (sub-cell ID) of the relay device 30A for which
connection is recommended. Accordingly, a process of connecting the
communication terminal 20 and the relay device 30A is performed,
and thus, it becomes possible for the communication terminal 20 to
communicate with the base station 10A via the relay device 30A.
Note that the steps of S462, S466, and S470 can be omitted in the
case of an autonomous operation or a decentralized operation in
which the management server 16 is not needed. In addition, although
FIG. 17 shows an example in which "Adjacent eNB & RN specific
context information" is transmitted from the relay device 30A, it
can be transmitted directly from the base station 10A to the
communication terminal 20.
[0203] FIG. 18 is a sequence diagram showing the procedures for
handing over the relay device 30. In the example shown in FIG. 18,
the communication terminal 20 is connected to the relay device 30A
belonging to the base station 10A. In this case, the relay device
30A transmits to the communication terminal 20 context information
(Adjacent eNB & RN context information) indicating the target
to be measured such as an adjacent base station or a nearby relay
device 30 (S504). After that, the communication terminal 20
measures the radio field intensity or the like of a signal
transmitted from the base station 10B or the like in accordance
with the context information while at the same time communicating
with the relay device 30A. Then, the communication terminal 20
reports the measurement information to the base station 10A via the
relay device 30 (S508, S512).
[0204] Herein, the measurement information can include the
interfered sub-carriers, resource blocks, the center frequency or
bandwidth, ID of the interfering node, link ID (ID indicating any
of the direct link, the access link, and the relay link), the
interference level or SINR level of each sub-carrier or resource
block, and the like.
[0205] After that, the base station 10A reports the measurement
information (Relay link information report) to the management
server 16 (S516), and the management server 16 transmits to the
base station 10A a confirmation in response to the measurement
information (S520). Then, if the management server 16 has
determined that the communication of the relay device 30A would
interfere with another communication on the basis of the reported
measurement information or a variety of other information, the
management server 16 transmits to the base station 10A link
management information about interference avoidance control (S524).
Herein, examples of the information about interference avoidance
control include ID of a relay device that performs communication
interfering with the relay device 30A, a channel used, the maximum
transmission power, positional information, and scheduling
information.
[0206] The base station 10A requests the base station 10B to, on
the basis of the link management information about interference
avoidance control received from the management server 16, accept
handover of the relay device 30A (S528), and once the base station
10B has accepted the request (S532), the base station 10A instructs
execution of handover of the relay device 30A (S536). Then, the
relay device 30A performs a process of connecting to the base
station 10B (S540), and informs the base station 10B that
preparation for handover is ready (S544). The base station 10B
returns ACK in response to the information (S548), and also reports
to the management server 16 that the relay device 30A has been
handed over to the base station 10B (S552).
[0207] Herein, the relay device 30A can be in a multi-link
connection state in which the relay device 30A is connected to both
the base station 10A and the base station 10B. In such a case, the
relay device 30A can switch the relay link to the base station 10B
only in the relay communication through the access link of the
communication terminal 20. Consequently, as the communication
terminal 20 belongs to the base station 10B, it becomes possible
for the base station 10B to centrally control the interference
avoidance between the communication terminals belonging to the base
station 10B, including the communication terminal 20.
[0208] Note that the relay device 30 can also generate a signal for
the management server 16 in accordance with the format of S1-MMEIF
or S1-UIF, and wirelessly transmit it to the base station 10. In
this case, the base station 10 can allow a signal received from the
relay device 30 to tunnel through to the management server 16.
Thus, as the connection relationship between the relay device 30
and the management server 16 becomes equivalent to the direct
connection, it is possible to increase the efficiency of the
centralized control of the management server 16. In addition,
although FIG. 18 shows an example in which the "Mesurement report"
is transmitted from the communication terminal 20 to the relay
device 30A in S508, the "Mesurement report" can be transmitted
directly from the communication terminal 20 to the base station
10A. Likewise, although the bottom portion in FIG. 18 shows an
example in which the relay device 30A transmits the "Mesurement
report" transmitted from the communication terminal 20 to the base
station 10B, the communication terminal 20 can transmit the
"Mesurement report" directly to the base station 10B. Further,
although FIG. 18 shows an example in which the "Adjacent eNB &
RN specific context information" is transmitted from the relay
device 30A, it can be transmitted directly from the base station
10A to the communication terminal 20.
[0209] Further, as another example of handover, if it is determined
that interference would be avoided by handing over the
communication terminal 20 belonging to the base station 10 to
another relay device 30 belonging to the same base station 10, the
base station management unit 166 instructs the base station 10 to
perform the handover. In that case, the base station management
unit 166 informs the base station 10 of the ID of the relay device
30, which is the handover destination, information for connection,
and the like. In response, a series of operations for handover is
performed. Hereinafter, a flow of the handover of the communication
terminal 20 will be specifically described with reference to FIG.
19.
[0210] FIG. 19 is a sequence diagram showing the procedures for
handing over the communication terminal 20. In the example shown in
FIG. 19, the relay devices 30A and 30X belong to the base station
10A, and the communication terminal 20 is connected to the relay
device 30A. In addition, as the processes in S554 to S570 in FIG.
19 are substantially the same as those in S504 to S520 shown in
FIG. 18, the detailed description thereof will be omitted.
[0211] If the management server 16 has determined on the basis of
the measurement information received from the base station 10A in
S566 or a variety of other information that interference to the
communication of the communication terminal 20 would be eliminated
by handing over the communication terminal 20 to the relay device
30X, the management server 16 issues an instruction to hand over
the communication terminal 20 to the relay device 30X using link
management information (S574).
[0212] The base station 10A requests the relay device 30X to, on
the basis of the link management information received from the
management server 16, accept handover of the communication terminal
20 (S578), and once the relay device 30X has accepted the request
(S582), the base station 10A instructs execution of handover of the
communication terminal 20 via the relay device 30A (S584, S586).
Then, the communication terminal 20 performs a process of
connecting to the relay device 30X (S590), and informs the base
station 10A that preparation for handover is ready via the relay
device 30X (S592, S594). Then, the base station 10A reports to the
management server 16 that the communication terminal 20 has been
handed over to the relay device 30X (S596). Although FIG. 19 shows
an example in which the "Adjacent eNB & RN specific context
information" is transmitted from the relay device 30A, it can be
transmitted directly from the base station 10A to the communication
terminal 20. In addition, although FIG. 19 shows an example in
which the "Mesurement report" is transmitted from the communication
terminal 20 to the relay device 30A, the "Mesurement report" can be
transmitted directly from the communication terminal 20 to the base
station 10A. Further, the steps of S566, S570, and S574 can be
omitted. Furthermore, the "Relay connection command" in S586 can be
transmitted directly from the base station 10A to the communication
terminal 20. Although FIG. 19 shows an example in which the
"Mesurement report" transmitted from the communication terminal 20
is relayed by the relay device 30X to the base station 10A, the
communication terminal 20 can transmit the "Mesurement report"
directly to the base station 10A. In addition, the "Adjacent eNB
& RN specific context information" can be transmitted from not
the relay device 30X but the base station 10A.
[0213] Further, as another example of handover, if it is determined
that interference would be avoided by handing over the
communication terminal 20 belonging to one of the base stations 10
to the relay device 30 belonging to the other base station 10, the
base station management unit 166 instructs the one of the base
stations 10 to perform the handover. In that case, the base station
management unit 166 informs the base station 10 of the ID of the
relay device 30, which is the handover destination, information for
connection, and the like. In response, a series of operations for
handover is performed. Hereinafter, a flow of the handover of the
communication terminal 20 will be specifically described with
reference to FIG. 20.
[0214] FIG. 20 is a sequence diagram showing the procedures for
handing over the communication terminal 20. In the example shown in
FIG. 20, the relay device 30A belongs to the base station 10A, the
relay device 30B belongs to the base station 10B, and the
communication terminal 20 is connected to the relay device 30A. As
the processes in S604 to S620 in FIG. 20 are substantially the same
as those in S504 to S520 shown in FIG. 18, the detailed description
thereof will be omitted.
[0215] If the management server 16 has determined on the basis of
the measurement information received from the base station 10A in
S616 or a variety of other information that communication of the
communication terminal 20 would interfere with communication of the
relay device 30B, the management server 16 issues an instruction to
hand over the communication terminal 20 to the relay device 30B
using link management information (S624).
[0216] The base station 10A requests the base station 10B to, on
the basis of the link management information received from the
management server 16, accept handover of the communication terminal
20 to the relay device 30B (S628), and once the base station 10B
has accepted the request (S632), the base station 10A returns ASK
(S636).
[0217] After that, the base station 10B inquires of the relay
device 30B if it is able to accept the handover (S640). Then, if
the relay device 30B is able to accept the handover (S644), the
base station 10B informs the relay device 30A that the relay device
30B is able to accept the handover, via the base station 10A (S648,
S652). Thus, such information is not transmitted when traffic in
the relay device 30B is congested or when there are no extra
resources available.
[0218] Then, when the relay device 30A has instructed the
communication terminal 20 to connect to the relay device 30B
(S656), the communication terminal 20 performs a process of
connecting to the relay device 30B (S660), and informs the relay
device 30B that preparation for handover is ready (S664). Then, the
relay device 30B transmits the information to the base station 10B
(S668), and the base station 10B transmits it to the base station
10A (S672). Then, the base station 10A reports to the management
server 16 that the communication terminal 20 has been handed over
to the relay device 30B (S676). Although FIG. 20 shows an example
in which the "Adjacent eNB & RN specific context information"
is transmitted from the relay device 30A, it can be transmitted
directly from the base station 10A to the communication terminal
20. In addition, although FIG. 20 shows an example in which the
"Mesurement report" is transmitted from the communication terminal
20 to the relay device 30A in S608, the "Mesurement report" can be
transmitted directly from the communication terminal 20 to the base
station 10A. Further, the "Relay connection command" in S652 can be
transmitted directly from the base station 10A to the communication
terminal 20.
[0219] Meanwhile, the base station management unit 166 can, when
traffic in one of the base stations, which controls the
communication for which it has been determined that interference
would occur, is congested to the extent that the scheduling
information cannot be changed, instruct the one of the base
stations 10 to prohibit the use of the relay device 30, which is
the cause of the interference, as the interference avoidance
control. The prohibition of the use of the relay device 30 is
instructed when, for example, resources allocated to relay devices
30 belonging to adjacent, different base stations 10 overlap, or
when there exists a communication terminal 20 between the adjacent,
different base stations 10 that belongs to each of the base
stations 10.
[0220] Alternatively, if it is determined that interference would
be avoided by adjusting a control parameter on the basis of
location-related information or scheduling information received
from each base station 10, the base station management unit 166 can
determine a control parameter for the communication controlled by
one of the base stations 10, and instruct the use of the determined
control parameter as the interference avoidance control. Herein,
examples of the control parameter include parameters related to the
transmission power, beam forming, transmission timing, a change in
the guard intervals, and insertion of a non-transmission section.
The base station 10, upon receiving the control parameter from the
management server 16, informs the relay device 30 of the control
parameter. Then, the relay device 30 performs communication through
the relay link and the access link in accordance with the control
parameter determined by the management server 16. Hereinafter, a
specific example of the determination of a control parameter will
be described with reference to the drawings.
[0221] FIG. 21 is an explanatory diagram showing a specific example
of the determination of the transmission power. In the example
shown in the upper view of FIG. 21, the relay device 30A belongs to
the base station 10A, the communication terminal 20A belongs to the
relay device 30A, the relay device 30B belongs to the base station
10B, and the communication terminal 20B belongs to the relay device
30B. In addition, the communication terminal 20B is included not
only in the radio wave coverage 32B of the relay device 30B but
also in the radio wave coverage 32A of the relay device 30A. Thus,
the interference determination unit 164 of the management server 16
determines that a signal transmitted from the relay device 30A to
the communication terminal 20A and a signal transmitted from the
relay device 30B to the communication terminal 20B would interfere
with each other at the communication terminal 20B.
[0222] In this case, the base station management unit 166
determines the transmission power that can avoid interference as
the transmission power of a signal from the relay device 30A to the
communication terminal 20A. Specifically, as shown in the lower
view of FIG. 21, the base station management unit 166 reduces the
transmission power so that the communication terminal 20B will not
be included in the radio wave coverage 32A of the signal
transmitted from the relay device 30A to the communication terminal
20A. Accordingly, interference caused by the relay device 30A can
be avoided.
[0223] FIG. 22 is an explanatory diagram showing a specific example
of the determination of beam forming. In the example shown in the
upper view of FIG. 22, the relay device 30A belongs to the base
station 10A, the communication terminal 20A belongs to the relay
device 30A, the relay device 30B belongs to the base station 10B,
and the communication terminal 20B belongs to the relay device 30B.
In addition, the communication terminal 20B is included not only in
the radio wave coverage 32B of the relay device 30B but also in the
radio wave coverage 32A of the relay device 30A. Thus, the
interference determination unit 164 of the management server 16
determines that a signal transmitted from the relay device 30A to
the communication terminal 20A and a signal transmitted from the
relay device 30B to the communication terminal 20B would interfere
with each other at the communication terminal 20B.
[0224] In this case, the base station management unit 166
determines that beam forming should be performed so that the signal
transmitted from the relay device 30A to the communication terminal
20A would not cause interference. Specifically, as shown in the
lower view of FIG. 22, the base station management unit 166 causes
beam forming to be performed so that the communication terminal 20B
will not be included in the radio wave coverage 32A of the signal
transmitted from the relay device 30A to the communication terminal
20A. In this manner, interference caused by the relay device 30A
can also be avoided by beam forming.
[0225] FIGS. 23 to 25 are explanatory diagrams showing specific
examples of the determination of the transmission timing, insertion
of a non-transmission section, and the like. In the example shown
in FIG. 23, the communication terminal 20B is included in the
signal coverage of the base station 10A and the relay device 30B.
Herein, even when the base station 10A and the relay device 30B
transmit signals using slots that are temporally orthogonal to each
other as shown in FIG. 24, there may be cases in which the
reception times at the communication terminal 20B may overlap.
Specifically, FIG. 24 shows an example in which the former half of
a signal transmitted from the base station 10A and the latter half
of a signal transmitted from the relay device 30B interfere with
each other.
[0226] In this case, as shown in FIG. 25, the base station
management unit 166 can delay the signal transmission timing of the
base station 10A. Alternatively, the base station management unit
166 can set a few head OFDM symbols of a signal transmitted from
the base station 10A as a non-transmission section, or lengthen
the
[0227] GIs. As a further alternative, the base station management
unit 166 can advance the transmission timing of the relay device
30B. As described above, it is also possible to avoid interference
by adjusting the transmission timing, inserting a non-transmission
section, or the like according to circumstances.
[0228] As described above, the base station management unit 166 can
instruct execution of various types of interference avoidance
control. Further, each base station 10 reports to the management
server 16 the execution process of the interference avoidance
control or communication quality information after the execution,
and the base station management unit 166 adequately adjusts a
control parameter in accordance with the reported communication
quality information. Note that each base station 10 can, when
requested for communication quality information from the management
server 16, report the communication quality information to the
management server 16 as soon as the preparation is ready.
[0229] For example, the base station management unit 166 can issue
an instruction to, when the number of generations of HARQ (Hybrid
Automatic Repeat Request) reported from the base station 10 is
greater than or equal to a prescribed number or when a packet loss
is greater than or equal to a predetermined level, increase the
transmission power using TPC of the relevant link.
[0230] In addition, in the arrangement shown in FIG. 21, when a
signal from the relay device 30B to the base station 10B interferes
with a signal from the communication terminal 20A to the relay
device 30A, the base station management unit 166 can issue
instruction to reduce the rate of a signal from the communication
terminal 20A to the relay device 30A or change the HARQ. Note that
exemplary methods of HARQ include Chase Combining and Incremental
Redundancy.
[0231] <4. Control Ranges B and C: Autonomous Control by the
Relay Device>
[0232] When the base station 10 has selected the control range B or
C, the relay device 30 is permitted to perform an autonomous
operation. Thus, the relay device 30 autonomously determines the
interference avoidance control, and executes the interference
avoidance control. Hereinafter, the autonomous operation performed
by the relay device 30 will be described.
[0233] (Information Supplied from Management Server 16)
[0234] The management server 16 supplies the following information
to the relay device 30, which has been determined to cause
interference by the interference determination unit 164, via the
base station 10.
[0235] Information about the locations of a relay device 30 and a
communication terminal belonging to an adjacent base station 10
that controls the communication interfering with the relay device
30. Note that this information includes information in both the
case in which the communication of the relay device 30 interferes
and the case in which the communication of the relay device 30 is
interfered.
[0236] ID, Qos information, and scheduling information of a relay
device 30 and a communication terminal belonging to an adjacent
base station 10 that controls the communication interfering with
the relay device 30. Note that when the relay device 30 is not
synchronous with the adjacent base station 10, reference counter
information for detecting a deviation in the synchronization is
also included.
[0237] Herein, the management server 16 can also select and supply
only part of the aforementioned location-related information and
scheduling information. Further, the management server 16 can also
inform the relay device 30 of the recommended interference
avoidance control (e.g., a control parameter).
[0238] The relay device 30, on the basis of the aforementioned
information supplied from the management server 16, determines and
executes the interference avoidance control. Examples of the
interference avoidance control include handover and link
adaptation. Hereinafter, such interference avoidance control will
be described in detail.
[0239] (Interference Avoidance Control: Handover)
[0240] FIG. 26 is an explanatory diagram showing a specific example
of handover of the relay device 30. In the example shown in the
upper view of FIG. 26, the relay device 30A belongs to the base
station 10A, the communication terminal 20A belongs to the relay
device 30A, and the communication terminal 20B belongs to the base
station 10B. Note that the relay device 30A can grasp the cell
structure shown in the upper view of FIG. 26 on the basis of the
location-related information supplied from the management server
16.
[0241] In the example shown in the upper view of FIG. 26, there are
cases in which, when the communication terminal 20B transmits a
signal to the base station 10B through the direct link UL at the
same time as when the relay device 30A transmits a signal to the
base station 10A through the relay link UL, both the signals may
interfere with each other at the base station 10B. Herein, the
control unit 342 of the relay device 30A can refer to the
scheduling information of the base station 10B supplied from the
management server 16, and execute the handover of the relay device
30A to the base station 10B if the base station 10B has extra
available resources for accepting the handover.
[0242] Accordingly, as shown in the lower view of FIG. 26, the
relay device 30A is connected to the base station 10B, and thus
belongs to the base station 10B. When the relay device 30A belongs
to the base station 10B, the base station 10B performs scheduling
so that the communication terminal 20B and the relay device 30A
will not interfere with each other. Thus, it is possible to avoid
mutual interference between a signal transmitted from the relay
device 30A and a signal transmitted from the communication terminal
20B.
[0243] Note that handover of the relay device 30A can be executed
on the basis of the measurement information reported from the
communication terminal 20A. Hereinafter, a variation of the
connection procedures of the communication terminal 20A and the
relay device 30A will be described first, and then, handover
procedures will be described.
[0244] FIG. 27 is a sequence diagram showing a variation of the
connection procedures of the communication terminal 20A and the
relay device 30A. The communication terminal 20A transmits an RRC
connection request to the relay device 30A using resources
allocated by the relay device 30A (S704). The relay device 30A,
upon receiving the RRC connection request from the communication
terminal 20A, requests the base station 10A to allocate resources
for the relay link and the access link (S708). If the base station
10A is able to allocate the resources requested from the relay
device 30A, the base station 10A transmits to the relay device 30A
information to the effect that the allocation is possible as well
as the resources to be allocated (S712).
[0245] Next, after the relay device 30A has transmitted ACK to the
base station 10A (S716), the relay device 30A transmits an RRC
connection resolution indicating the source of transmission of the
RRC connection request (S720). Then, the base station 10A transmits
to the management server 16 a connection request indicating that
the communication terminal 20A is requesting a service (S724). The
management server 16, upon receiving the connection request,
transmits information for performing setup on the communication
terminal 20 through connection setup (S728).
[0246] Then, the base station 10 transfers the connection setup
from the management server 16 to the relay device 30A (S732), and
the relay device 30A transmits RRC connection setup to the
communication terminal 20A (S736), and then the communication
terminal 20A performs connection setup. After that, the
communication terminal 20A transmits to the relay device 30A RRC
connection complete indicating that the connection setup is
complete (S740). Accordingly, the communication terminal 20A and
the relay device 30A are connected, whereby it becomes possible for
the communication terminal 20A to communicate with the base station
10A via the relay device 30A.
[0247] FIG. 28 is a sequence diagram showing the procedures for
handing over the relay device 30A. In the example shown in FIG. 28,
the relay device 30A belongs to the base station 10A, and the base
station 20A and the relay device 30A are connected. In this case,
the relay device 30A transmits to the communication terminal 20A
context information (Adjacent eNB & RN context information)
indicating the target to be measured such as an adjacent base
station, a nearby relay device 30, or the like (S754). After that,
the communication terminal 20A measures the radio field intensity
or the like of a signal transmitted from the base station 10B or
the like in accordance with the context information while at the
same time communicating with the relay device 30A. Then, the
communication terminal 20A reports the measurement information to
the relay device 30A (S758).
[0248] Next, if the relay device 30A has determined on the basis of
the measurement information received from the communication
terminal 20A, information from the management server 16, or the
like that handover to the base station 10B would be effectively
performed while avoiding interference, the relay device 30A
performs a process of connecting to the base station 10B (S762).
Herein, while the relay device 30A is performing a connection
process, it is difficult for the relay device 30A to relay the
communication with the communication terminal 20A. Thus, if the
relay device 30A has transmission/reception resources (e.g., a
plurality of antennae) with which a plurality of processes can be
performed in parallel, it is possible to use some of the
transmission/reception resources to communicate with the
communication terminal 20A, and use the other of the
transmission/reception resources to perform the process of
connecting to the base station 10B. Alternatively, the relay device
30A can cause the communication terminal 20A to be directly
connected to the base station 10A and, when the process of
connecting to the base station 10B is completed, return the
communication terminal 20A to a position belonging to the relay
device 30A.
[0249] After that, the relay device 30A performs measurement on the
basis of the context information received from the base station
10B, and transmits the measurement information to the base station
10B (S766). In addition, the relay device 30A transmits context
information to the communication terminal 20A and receives
measurement information, which has been obtained through
measurement by the communication terminal 20A, from the
communication terminal 20A (S770).
[0250] Meanwhile, as described below, there are also cases in which
interference can be avoided by handing over not the relay device 30
but the communication terminal 20.
[0251] FIG. 29 is an explanatory diagram showing a specific example
of handover of the communication terminal 20. In the example shown
in the upper view of FIG. 29, the relay device 30A belongs to the
base station 10A, the communication terminal 20A belongs to the
relay device 30A, and the relay device 30B and the communication
terminal 20B belong to the base station 10B.
[0252] In the example shown in the upper view of FIG. 29, there are
cases in which, when the communication terminal 20B transmits a
signal to the base station 10B via the direct link UL at the same
time as when the relay device 30A transmits a signal received from
the communication terminal 20A to the base station 10A via the
relay link UL, both the signals may interfere with each other at
the base station 10B. Herein, the control unit 342 of the relay
device 30A can refer to the scheduling information of the base
station 10B supplied from the management server 16, and execute the
handover of the communication terminal 20A to the base station 10B
if the base station 10B has extra available resources for accepting
the handover.
[0253] Specifically, the relay device 30A can terminate the
connection with the communication terminal 20A. This is because it
is considered that the communication terminal 20A will attempt
connection with the base station 10B thereafter. Alternatively, the
relay device 30A can explicitly request for handover of the
communication terminal 20A to the base station 10B or the relay
device 30B.
[0254] As shown in the lower diagram of FIG. 29, when the
communication terminal 20A is handed over to the relay device 30B,
a signal transmitted from the communication terminal 20A will no
more be relayed by the relay device 30A. Thus, interference shown
in the upper view of FIG. 20 can be avoided. Note that the relay
device 30A can also control the handover of the communication
terminal 20 when the number of the communication terminals 20
belonging to the relay device 30A is greater than or equal to a
predetermined number (when the number that can be handled is
approaching the limit). Alternatively, the relay device 30A can
also select as the target to be handed over the communication
terminal 20 whose access link CQI does not meet a predetermined
standard.
[0255] Note that the relay device 30A can also execute the handover
on the basis of the measurement information reported from the
communication terminal 20A. Hereinafter, the procedures for handing
over the communication terminal 20A will be described with
reference to FIG. 30.
[0256] FIG. 30 is a sequence diagram showing the procedures for
handing over the communication terminal 20A. In the example shown
in FIG. 30, the relay device 30A belongs to the base station 10A,
the relay device 30B belongs to the base station 10B, and the
communication terminal 20A is connected to the relay device 30A. In
this case, the relay device 30A transmits to the communication
terminal 20A context information indicating the target to be
measured such as an adjacent base station, a nearby relay device
30, and the like (S804). After that, the communication terminal 20A
measures the radio field intensity or the like of a signal
transmitted from the base station 10B, the relay device 30B, or the
like in accordance with the context information while at the same
time communicating with the relay device 30A. Then, the
communication terminal 20A reports the measurement information to
the relay device 30A (S808).
[0257] Next, it is assumed that the relay device 30A has determined
on the basis of the measurement information received from the
communication terminal 20A, information from the management server
16, or the like that handover of the communication terminal 20A to
the relay device 30B would be effectively performed while avoiding
interference. In this case, the relay device 30A requests for
handover of the communication terminal 20A to the relay device 30B
to the base station 10B via the base station 10A (S812, S816).
Then, the base station 10B transmits to the base station 10A a
confirmation in response to the handover request (S820), and
receives ACK from the base station 10A (S824).
[0258] Then, the base station 10B inquires of the relay device 30B
if it is able to accept the handover (S828). Then, if the relay
device 30B is able to accept the communication terminal 20A (S832),
the base station 10B informs the relay device 30A that the relay
device 30B is able to accept the handover, via the base station 10A
(S836, S840).
[0259] Then, the relay device 30A transmits context information and
a signal recommending the handover to the relay device 30B (S844,
S848). Further, the relay device 30A requests the communication
terminal 20A to cancel the connection with the relay device 30A
(S852), and, upon receiving a confirmation in response to the
connection cancelation from the communication terminal 20A (S856),
returns ACK to the communication terminal 20A (S860). Accordingly,
the connection between the communication terminal 20A and the relay
device 30A is cancelled, and the communication terminal 20A
performs a process of connecting to the relay device 30B that is
the recommended handover destination (S864).
[0260] Although the description has been made above of an example
in which both the recommendation of the handover to the relay
device 30B and requesting for cancellation of the connection with
the relay device 30A are performed, one or both of them need not be
performed. For example, the relay device 30A can forcibly cancel
the connection with the communication terminal 20B without
performing each of the aforementioned processes. In that case, it
is expected that the communication terminal 20B voluntarily
performs a process of connecting to the base station 10 or the
relay device 30 included in the context information.
[0261] In addition, although the description has been made above of
an example in which the communication terminal 20A is handed over
to the relay device 30B belonging to a different base station, the
communication terminal 20A can also be handed over to the relay
device 30X belonging to the same base station 10A as described
below.
[0262] FIG. 31 is a sequence diagram showing the procedures for
handing over the communication terminal 20A. In the example shown
in FIG. 31, the relay device 30A and the relay device 30X belong to
the base station 10A, and the communication terminal 20A is
connected to the relay device 30A. The relay device 30A receives
measurement information from the communication terminal 20A (S904),
and determines the interference avoidance control for the
communication of the communication terminal 20A, on the basis of
the measurement information, information supplied from the
management server 16, or the like. Herein, if the relay device 30A
has determined that handover of the communication terminal 20A to
the relay device 30X would be effectively performed while avoiding
interference, the relay device 30A transmits context information
and a signal recommending the handover to the relay device 30X
(S908, S912).
[0263] Further, when the relay device 30A requests the
communication terminal 20A to cancel the communication with the
relay device 30A (S916) and receives a confirmation in response to
the connection cancellation from the communication terminal 20A
(S920), the relay device 30A returns ACK to the communication
terminal 20A (S924). Accordingly, the connection between the
communication terminal 20A and the relay device 30A is cancelled,
and the communication terminal 20A performs a process of connecting
to the relay device 30X that is the recommended handover
destination (S928).
[0264] As described above, the relay device 30 can avoid
interference by executing handover to the adjacent base station 10
or by handing over the communication terminal 20 belonging to the
relay device 30 to another relay device 30.
[0265] (Interference Avoidance Control: Link Adaptation)
[0266] There are cases in which the relay device 30 can avoid
interference by performing link adaptation when it is informed by
the management server 16 of a slot that has a possibility of
generating interference, or of the ID, location-related
information, and permissible interference level of the relay device
30, the communication terminal 20, or the base station 10 to
interfere with or to be interfered with. Examples of the link
adaptation for the access link that can be controlled by the relay
device 30 include TPC, AMC (Advanced Modulation Control), and HARQ.
Hereinafter, each link adaptation will be specifically
described.
[0267] The relay device 30, when instructed by the management
server 16 or the like to suppress the level of interference to
another communication, or when having determined that the level of
interference to another communication should be suppressed,
executes any of the following link adaptation. [0268] (1) Lower the
transmission power, and improve the reception SNIR using HARQ.
[0269] (2) Lower the transmission power, and lower the necessary
SNIR by lowering the Modulation and Coding rate.
[0270] The relay device 30 needs additional resources when either
of (1) or (2) above is executed. Thus, the relay device 30, when
allocated extra resources in advance from the base station 10, uses
the extra resources, and when running short of extra resources,
requests the base station 10 or the management server 16 to
allocate resources. Note that the base station 10 or the management
server 16, when requested to allocate resources to avoid
interference, prioritizes the resource allocation over other
requests.
[0271] Meanwhile, when the relay device 30 performs communication
even through the level of interference from another communication
is high, the relay device 30 executes any of the following link
adaptation. [0272] (3) Increase the transmission power. [0273] (4)
Improve the reception SNIR using HARQ. [0274] (5) Lower the
necessary SN IR by lowering the Modulation and Coding rate.
[0275] In order to execute (4) and (5) above, additional resources
are needed. Thus, the relay device 30, when allocated extra
resources in advance from the base station 10, uses the extra
resources, and when running short of extra resources, requests the
base station 10 or the management server 16 to allocate resources.
Note that the base station 10 or the management server 16, when
requested to allocate resources to avoid interference, prioritizes
the resource allocation over other requests.
[0276] In OFDMA, link adaptation can be performed in units of
resource blocks or sub-carriers. Thus, the relay device 30 can
execute the link adaptation shown in (1) to (5) above to only the
sub-carrier or resource block whose interference level is above a
predetermined level. Specifically, the relay device 30 can, when
transmitting a signal using a resource block A whose level of
interference from another communication is above the predetermined
level and a resource block B whose level of interference from
another communication is below the predetermined level, execute any
of (3) to (5) above only to the resource block A.
[0277] <5. Other Examples of Application of the Present
Invention>
[0278] The description has been made above of selecting the control
range granted for the relay device 30 from among a plurality types
of control range, implementing centralized control with the
management server 16 for avoiding interference between cells formed
by the base stations 10, and executing interference avoidance
control with the relay device 30 by autonomously determining the
interference avoidance control. However, the aforementioned relay
device 30 is merely an example of a small-to-medium-sized base
station in a heterogeneous network described below.
[0279] That is, the following also fall within the technical scope
of the present invention: selecting the control range granted for a
small-to-medium-sized base station from among a plurality of types
of control range, implementing centralized control with the
management server 16 for avoiding interference between cells formed
by the base stations 10 or small-to-medium-sized base stations, and
executing interference avoidance control with a
small-to-medium-sized base station by autonomously determining the
interference avoidance control.
[0280] A heterogeneous network is a network in which a plurality of
types of small-to-medium-sized base stations coexists within a
macrocell by performing overlay transmission or spectrum sharing.
Examples of small-to-medium-sized base stations include an RRH
(Remote RadioHeaD) cell base station, a hot zone base station
(Pico/micro cell eNB), a femtocell base station (Home eNB), and a
relay device (relay base station). Hereinafter, the configuration
of the heterogeneous network will be specifically described.
[0281] FIG. 32 is an explanatory diagram showing a configuration
example of a heterogeneous network. As shown in FIG. 32, the
heterogeneous network includes a macrocell base station 10 (which
is synonymous with the base station 10), a relay device 30, a hot
zone base station 31, a femtocell base station 32, an RRH cell base
station 33, and management servers 16A and 16B.
[0282] The management servers 16A and 16B have functions with which
the macrocell base station 10 and the small-to-medium-sized base
stations operate cooperatively. For example, as described in "3.
Control Range A: Centralized Control by the Management Server," the
management server 16A receives information about the macrocell base
station 10, the small-to-medium-sized base stations, and the
communication terminals 20 belonging to the small-to-medium-sized
base stations (positional information, scheduling information, Qos
information, and the like), and determines the macrocell base
station 10 or the small-to-medium-sized base station that controls
the communication interfering with another communication, and
further issues an instruction to execute an interference avoidance
operation. Note that the function of the management server 16 can
also be implemented by the macrocell base station 10 or the
small-to-medium-sized base stations.
[0283] The macrocell base station 10 manages the
small-to-medium-sized base stations and the communication terminals
20 within the macrocell. For example, as described in "2-3.
Configuration of the Base Station," the macrocell base station 10
selects the control range granted for each small-to-medium-sized
base station from among the control range A, the control range B,
and the control range C. Then, each small-to-medium-sized base
station controls the communication with the communication terminal
20 in accordance with the control range selected by the macrocell
base station 10.
[0284] The hot zone base station 31 (a picocell base station or a
microcell base station) has a lower maximum transmission power than
the macrocell base station 10, and communicates with the macrocell
base station 10 using an interface such as X2 or S1 of a core
network. Note that the hot zone base station 31 forms an OSG (Open
Subscriber Group) that is accessible from any communication
terminal 20.
[0285] The femtocell base station 32 has a lower maximum
transmission power than the macrocell base station 10, and
communicates with the macrocell base station 10 using a
packet-switched network such as ADSL. Further, the femtocell base
station 32 can also communicate with the macrocell base station 10
through a wireless link. Note that the femtocell base station 32
forms a CSG (Closed Subscriber Group) that is accessible from only
the limited communication terminals 20.
[0286] The RRH cell base station 33 is connected to the macrocell
base station 10 through an optical fiber. Therefore, the macrocell
base station 10 can transmit signals to the RRH cell base stations
33A and 33B arranged in different geographical locations via
optical fibers, and can cause the RRH cell base stations 33A and
33B to wirelessly transmit signals. For example, the macrocell base
station 10 can use only the RRH cell base station 33 located near
the communication terminal 20. Note that the function of the
control system is implemented by the macrocell base station 10, and
an optimum transmission form is selected in accordance with the
distribution of the communication terminals 20.
[0287] FIG. 33 shows the outline of each small-to-medium-sized base
station described above. The small-to-medium-sized base station
such as the hot zone base station 31 or the femtocell base station
32 can autonomously determine the interference avoidance control
and execute the determined interference avoidance control in
accordance with the method described in "4. Control Ranges B and C:
Autonomous Control by the Relay Device." Hereinafter, an
interference model in a heterogeneous network and interference
avoidance control will be described.
[0288] (Interference Model in Heterogeneous Network)
[0289] FIG. 34 is an explanatory diagram showing an interference
model in a heterogeneous network. Note that in FIG. 34 and FIGS. 35
to 37 described below, the relay device 30, the hot zone base
station 31, the femtocell base station 32, and the like are not
particularly distinguished from each other, and are collectively
shown as small-to-medium-sized base stations 40.
[0290] As shown in FIG. 34, in a heterogeneous network, it is
supposed that interference as shown below will be generated. [0291]
(1) A case in which a signal transmitted from a
small-to-medium-sized base station 40A and a signal transmitted
from the macrocell base station 10 interfere with each other at a
communication terminal 20A-2. [0292] (2) A case in which a signal
transmitted from a communication terminal 20B-2 and a signal
transmitted from the macrocell base station 10 interfere with each
other at a small-to-medium-sized base station 40B. [0293] (3) A
case in which a signal transmitted from a small-to-medium-sized
base station 40C and a signal transmitted from the macrocell base
station 10 interfere with each other at a small-to-medium-sized
base station 40D. [0294] (4) A case in which a signal transmitted
from a small-to-medium-sized base station 40E and a signal
transmitted from a communication terminal 20E-2 interfere with each
other at a communication terminal 20E-2.
[0295] (Interference Avoidance Control in Heterogeneous
Network)
[0296] As described above, various types of interference are
generated in the heterogeneous network. However, such interference
can be addressed by performing the interference avoidance control
described in "3. Control Range A: Centralized Control by the
Management Server" or "4. Control Ranges B and C: Autonomous
Control by the Relay Device." Hereinafter, an example of the
interference avoidance control will be specifically described.
[0297] FIG. 35 is an explanatory diagram showing exemplary
interference avoidance performed through handover. In the left view
of FIG. 35, a signal transmitted from the small-to-medium-sized
base station 40A and a signal transmitted from the macrocell base
station 10 interfere with each other at the communication terminal
20A-2. In this case, the interference can be eliminated by handing
over the communication terminal 20A-2 from the
small-to-medium-sized base station 40A to the small-to-medium-sized
base station 40G whose transmission timing differs from that of the
macrocell base station 10.
[0298] In addition, in the left view of FIG. 35, a signal
transmitted from the small-to-medium-sized base station 40E and a
signal transmitted from the communication terminal 20E-2 interfere
with each other at the communication terminal 20E-2. In this case,
the interference can be eliminated by handing over the
communication terminal 20E-2 from the small-to-medium-sized base
station 40E to the small-to-medium-sized base station 40F.
[0299] Note that handover between the small-to-medium-sized base
stations 40 can be performed in accordance with the sequence of
handover of a relay device 30 to another relay device 30 belonging
to the same base station 10 described with reference to FIG. 19,
for example. Meanwhile, handover between the small-to-medium-sized
base stations 40 belonging to different macrocell base stations 10
can be performed in accordance with the sequence of handover
described with reference to FIG. 20, for example. Herein, there are
also cases in which the small-to-medium-sized base station 40 has
an interface for communicating directly with the management server
16. However, as the small-to-medium-sized base station 40 is under
the management of the macrocell base station 10, the
small-to-medium-sized base station 40 communicates with the
macrocell base station 10 for performing handover as shown in FIG.
19 and the like.
[0300] However, an interface between the macrocell base station 10
and the small-to-medium-sized base station 40 differs according to
the type of the small-to-medium-sized base station 40. For example,
when the small-to-medium-sized base station 40 is the hot zone base
station 31, the small-to-medium-sized base station 40 and the
macrocell base station 10 communicate with each other using an X2
interface. Alternatively, when an interface between the
small-to-medium-sized base station 40 and the macrocell base
station 10 is a wired interface, it is possible to use latency as
the criteria for determining the quality of the communication
link.
[0301] FIG. 36 is an explanatory diagram showing exemplary
interference avoidance performed through beam forming. In the left
view of FIG. 36, a signal transmitted from the
small-to-medium-sized base station 40A and a signal transmitted
from the macrocell base station 10 interfere with each other at the
communication terminal 20A-2. In this case, the communication
terminal 20A-2 can eliminate the interference by making its
reception directivity face the direction in which the
small-to-medium-sized base station 40A is arranged.
[0302] In addition, in the left view of FIG. 36, a signal
transmitted from the small-to-medium-sized base station 40 and a
signal transmitted from the communication terminal 20E-2 interfere
with each other at the communication terminal 20E-2. In this case,
if the communication terminal 20E-2 makes its transmission
directivity face the direction in which the small-to-medium-sized
base station 40F is arranged, the signal transmitted from the
communication terminal 20E-2 will no more reach the communication
terminal 20E, whereby the interference can be eliminated.
[0303] FIG. 37 is an explanatory diagram showing exemplary
interference avoidance performed through transmission power
control. In the left view of FIG. 37, a signal transmitted from the
small-to-medium-sized base station 40A and a signal transmitted
from the macrocell base station 10 interfere with each other at the
communication terminal 20A-2. In this case, if the transmission
power of the small-to-medium-sized base station 40A is lowered, the
communication terminal 20A-2 will be out of the radio wave coverage
of the small-to-medium-sized base station 40A. Thus, the connection
between the communication terminal 20A-2 and the
small-to-medium-sized base station 40A terminates. Accordingly, the
communication terminal 20A-2 searches for a new target to connect
to, and is then connected to the macrocell base station 10, for
example. Thus, lowering the transmission power of the
small-to-medium-sized base station 40A allows the interference to
be eliminated.
[0304] In addition, in the left view of FIG. 37, a signal
transmitted from the small-to-medium-sized base station 40 and a
signal transmitted from the communication terminal 20E-2 interfere
with each other at the communication terminal 20E-2. In this case,
if the transmission power of the small-to-medium-sized base station
40F is lowered, the communication terminal 20E-2 will be out of the
radio wave coverage of the small-to-medium-sized base station 40F.
Thus, the connection between the communication terminal 20E-2 and
the small-to-medium-sized base station 40F terminates. Accordingly,
the communication terminal 20E-2 searches for a new target to
connect to, and is then connected to the macrocell base station 10,
for example. Thus, lowering the transmission power of the
small-to-medium-sized base station 40F allows the interference to
be eliminated.
[0305] <6. Conclusion>
[0306] As described above, according to the present embodiment, the
control range granted for a small-to-medium-sized base station such
as the relay device 30 can be selected from among a plurality of
types of control range. In addition, according to the present
embodiment, the management server 16 can implement centralized
control for avoiding interference that would occur between cells
formed by the base stations 10. Further, according to the present
embodiment, a small-to-medium-sized base station such as the relay
device 30 can autonomously determine the interference avoidance
control and execute the interference avoidance control.
[0307] Although the preferred embodiments of the present invention
have been described in detail with reference to the appended
drawings, the present invention is not limited thereto. It is
obvious to those skilled in the art that various modifications or
variations are possible insofar as they are within the technical
scope of the appended claims or the equivalents thereof. It should
be understood that such modifications or variations are also within
the technical scope of the present invention.
[0308] For example, although an example in which the control range
granted for the relay device 30 is dynamically selected is
described above, the control range granted for the relay device 30
can be fixed. Thus, when the control range A is fixedly set on the
relay device 30, interference avoidance control is performed in
accordance with the procedures described in "3. Control Range A:
Centralized Control by the Management Server." Meanwhile, when the
control range B or C is fixedly set on the relay device 30,
interference avoidance control is performed in accordance with the
procedures described in "4. Control Ranges B and C: Autonomous
Control by the Relay Device."
[0309] The steps in the process of the communication system 1 in
this specification need not necessarily be processed in a
time-series order in accordance with the order described in the
sequence diagram. For example, the steps in the process of the
communication system 1 can be performed in an order different from
that described in the sequence diagram, or be processed in
parallel.
[0310] It is also possible to create a computer program for causing
built-in hardware in the base station 10, the management server 16,
and the relay device 30, such as a CPU, ROM, and RAM, to exert a
function that is equivalent to each of the aforementioned
configurations of the base station 10, the management server 16,
and the relay device 30. In addition, a storage medium having the
computer program stored therein is also provided.
* * * * *